I was just going to do a whole roasted filet but then I got the initial idea for the stuffed roast from watching American's Test Kitchen - the meat recipe is loosely based on their recipe.
Caramelized Onion/Gruyere Potato Gratin
4 lbs Yukon Gold "A" (medium sized potatoes - not the baby ones)
Reserved caramelized onions from recipe below
4 cups heavy cream
4 sprigs fresh thyme
4 cloves peeled garlic, smashed
2 shallots, roughly diced
1/2 t. freshly grated nutmeg
12 oz gruyere cheese, sliced or grated
4 oz. grated parmesan cheese (the real stuff, please!)
salt & pepper
4 oz. butter, soft
Combine the cream, thyme, nutmeg, garlic, shallots, a big pinch of salt & lots of freshly ground pepper - heat to a simmer and set aside to infuse while you prepare the rest of the gratin.
Pick two of the most uniformly round potatoes and set them aside - you are going to use these for your top layer. Slice the potatoes very thinly (here I am using a Benriner Japanese slicer - worth the $20.00 investment if you don't have one.) 
Butter a baking dish. Layer the potatoes - this is where you will use the irregularly shaped potatoes. 
Add half the caramelized onions and season well with salt and pepper.
Add cheese - I just happened to have sliced but grated is fine, too. Make another layer of potatoes, onion, seasoning and cheese. 
Slice the reserved potatoes and arrange them atop the gratin in even rows. Season the top with salt and pepper.
Strain the cream mixture and pour evenly over the gratin.
Sprinkle evenly with parmesan cheese, dot with butter. Bake at 350 for 1 hour and 1/2 hour. If it starts to brown too much, cover loosely with foil.
The finished gratin. If I had to do over, I probably would have put foil over it in the last half hour of cooking but those browned bits were actually quite delicious!Porcini Stuffed Beef Tenderloin
8 yellow onions
1/4 cup olive oil
4 oz. butter
1lb. frozen porcini mushrooms
1 cup madeira
1 T. chopped, fresh thyme
2 cloves garlic, chopped finely
1 whole beef tenderloin roast, 8lbs. (aka PSMO)
Salt, freshly ground pepper, olive oil
Compound Butter:
6 oz. Butter, soft
1/4 cup fresh parsley
2 t. fresh thyme, finely chopped
1 large clove garlic, finely chopped
1 T. whole grain mustard
lemon zest from 1/2 lemon
salt & pepper to taste
Combine all the ingredients for the compound butter in a food processor. Pulse until all the herbs are finely chopped and it is well combined. Taste for seasoning, adjust. Set the butter aside but keep it soft.
Peel and halve onions
Thinly slice the onions - when I have something this monotonous to do, I time myself to make it a game. This took me two minutes - a personal best! 
Heat a heavy bottomed pan. Add butter and olive oil; heat until butter is melted and foamy.
Add onions to the pan, it will be quite full. Carefully stir the onions to coat with the oil/butter mixture and leave it alone. . low and slow, stirring occasionally.
The onions are now opaque and soft. Season with salt and pepper; continue to cook, stirring occasionally.
The onions are done! Remove about 1/3 of the onions and set them aside. Keep heat at low while you prepare the porcinis.
I ended up using frozen porcinis because the fresh ones I could find were wormy and very expensive. While the thawed frozen ones are none too pretty, for this preparation they are perfect! 
Finely puree the porcinis in a food processor. Increase the heat on the onions to high and add the puree. 
At this point in the game, it's not pretty. Stir the onions and pureed mushrooms well, reduce the heat and let cook down.
Once the mixture is reduced as seen in the picture, add the madeira and reduce again.
Add garlic, thyme, salt & freshly ground pepper to taste. Saute briefly.
Spread the mixture evenly on a sheet pan and set aside to cool while you prepare the meat.
Here's the BEEF! 
A very brief lesson on Whole Beef Tenderloin. The most common is the PSMO (pismo) which the whole tenderloin, most of the fat removed, the side muscle intact (aka "chain") with silver skin still intact. We all know what a filet mignon is. This is the cut it originates from. The PSMO has a balled end and a tapered end. The larger end is known as the Butt Tenderloin and it's where larger filets are cut from. If you order a filet mignon at a restaurant and receive one that has what looks almost like two pieces, then that is a filet cut from the Butt (hee hee!). The middle is known as the Chateaubriand and this is the best part of the whole tenderloin it is sometimes served as a roast for two in restaurants. This is the part we are going to use for our roast. The tapered end is known as the Short Tenderloin. You get your petite filet mignons, tournedos and tenderloin tips from this part of the cut. Another well known preparation from this cut is from the butt tenderloin to just where the meat starts to taper off is called a Filet Roast. In this picture, you can see where I started to remove the silver skin toward the butt end. A lot of membranes/fat can be removed by hand but the silver skin is quite stubborn. It's a good practice to pierce the silver skin with the tip of a sharp knife and then run the knife in the other direction, AWAY from the meat and toward the silver skin. . .you want to be careful to remove only silver skin with very little meat. It takes practice and patience (this is where a tripod would have been monumentally helpful).
I like to run my hand along the tenderloin and pull off whatever membranes I can with my hand. You can pretty much remove the side muscle, or "chain", up to the Butt end, then you have to gingerly remove it with your knife. As you work your way up the chain, you will see a lot of silver skin and fibers where the meat connects at the Butt end. Carefully trim all that away. Save the chain meat to grind for burgers or if you have a spectacularly spoiled dog, cut it into little pieces and share the wealth with your canine friend.
I wanted only the chateaubriand but needed a larger piece than the average 2lb. yield. To achieve that, I cut up closer on the Butt end to get a large roast. My knife demarcates where a true Chateaubriand would end. Note the butt end, Short end and chain. Lots of meat left over. I made an amazing Beef Stroganoff the next day for Christmas Day supper - it was incredible!
Now, we are going to "book" the meat. As in open it like a book. When working with a piece of meat this expensive, it is somewhat daunting but go slowly, delicately guiding your knife with little force - let the sharpness of your knife do the work. You are going to open the meat into thirds. So first, starting about an inch from the bottom, start cutting towards the middle - go slowly and carefully. When you get to the middle stop without cutting all the way through. Carefully "open" the meat.
Take a deep breath. Let's book the next part. Again, guide your knife about 1 inch from the bottom of the thickest piece of the meat - stopping just as you reach the end of the roast. Carefully "open" the meat.
Phew! 
Season the meat liberally with salt and pepper.
Spread the cooled filling evenly over the meat. 
Arrange butcher's twine in eight 1-inch intervals on top of your cutting board. Gently but firmly, roll the roast into a compact cylinder. Some of the filling might squeeze out but that's okay. Tie the ends off first.
Continue to tie off the meat securely but not too tightly. At this point, you can chill the meat until you're ready to use it. Make sure to bring it to room temperature for 45 minutes before you brown it. 
Heat oil in a large pan. Season the outside of the meat liberally with salt and freshly ground pepper. 
Brown all four sides.
Heat oven to 450F. Place the roast on a rack and roast for 20 minutes for medium rare. Tent with foil and let rest for 15 minutes before slicing. Keep the string on and use it as a guide for slicing. Remove string after plating each slice. AND THIS IS WHERE I START TO CRY!! The little red button on my camera started flashing and it was done. :( Sorry, friends, that you don't get to see the final plate up. It was lovely. A nice square of the potato gratin in the center of the plate, sauteed savoy cabbage with little nueske's bacon lardon surrounding the gratin and in the center. . .a beautifully rare spiral of tenderloin with the compound butter dripping down the side. Seriously amazing.
