Part 1: Make the mushroom stuffing, shape it into a log, and freeze it.

The mushroom stuffing is a simple saute of various kinds of mushrooms. Heat oil and butter in the pan; add liver and saute. Remove it from pan and place in food processor bowl with sharp blade inserted. To the saute pan, add saute shallots and garlic and the diced mushrooms -- for the dish in thhe photo, I used a combination of shiitake, crimini, portabello and it came out very well. When brown, add this mixture to the food processor. Add chicken browth, parsley, cognac, lemon juice and lemon zest, and salt and pepper to taste. Blend until mixture forms a course paste (rather like tapenade). Cool completely, and roll onto plastic wrap. Shape in the form of a 1 1/2 inch diameter log, and freeze at least 4 hours, preferably overnight. This mixture can be made ahead up to 2 days in advance.

Part 2. Slice the tenderloin down the middle, lengthwise.

Part 3. Take frozen mushroom log, trim it to length, and insert it in the tenderloin. Wrap and tie tightly with kitchen string every inch. Very generously coat with salt and freshly cracked pepper.

Part 4. Place it in roasting pan and let it rest for at least 2 hours. Put meat thermometer in the roast, but do not poke the tip through the center. Oven-sear the roast, by first preheating the oven to 500 degrees F. Brown it in the oven for 15 minutes, then turn temperature down to 325. Roast until the meat thermometer reads 125F for rare, 130F for medium rare, which will be about 30 to 45 minutes more. Remove from oven and let it rest at least 15 minutes before cutting into 1 inch slices. Plate it on top of bearnaise sauce or port wine sauce, and garnish with parsley.