For the Duxelles:
3 pints (1 1/2 pounds) white button mushrooms
2 shallots, peeled and roughly chopped
4 cloves garlic, peeled and roughly chopped
2 sprigs fresh thyme, leaves only
2 tablespoons unsalted butter
2 tablespoons extra-virgin olive oil
Kosher salt and freshly ground black pepper
For the Beef:
1 (3-pound) center cut beef tenderloin (Filet Mignon)
Extra-virgin olive oil
Kosher salt and freshly ground black pepper
12 thin slices prosciutto
6 sprigs of fresh thyme, leaves only
2 tablespoons Dijon mustard
Flour, for rolling out puff pastry
1 pound puff pastry, thawed if using frozen
2 large eggs, lightly beaten
1/2 teaspoon coarse sea salt
Directions (preheat oven to 400)
To make the Duxelles: Add mushrooms, shallots, garlic, and thyme to a food processor and pulse until finely chopped. Add butter and olive oil to a large saute pan and set over medium heat. Add the shallot and mushroom mixture and saute for 8 to 10 minutes until most of the liquid has evaporated. Season with salt and pepper and set aside to cool.
To prepare the beef: Season the tenderloin with salt and pepper and sear all over, in a hot cast iron skillet. When the beef is seared, remove from heat, allow to cool slightly, and then smear lightly all over with Dijon mustard. Meanwhile get out a sheet of plastic wrap. Shingle the prosciutto on top of the plastic wrap so it forms a rectangle that is big enough to encompass the entire filet of beef. Then spread a thin layer of duxelles on top of the prosciutto. Season the surface of the duxelles with salt and pepper and sprinkle with fresh thyme leaves. Next place the beef on the duxelles covered prosciutto and roll up using the plastic wrap to tie it up nice and tight. Tuck in the ends of the prosciutto as you roll to completely encompass the beef. Set in the refrigerator for 30 minutes.
On a lightly floured surface, roll the puff pastry out to about a 1/4-inch thickness. Remove beef from refrigerator and take off the plastic. Set the beef in the center of the pastry and fold over the longer sides, brushing with egg wash to seal. Trim ends if necessary then brush with egg wash and fold over to completely seal the beef. Place the beef seam side down on a baking sheet. Brush the top of the pastry with egg wash then make a couple of slits in the top of the pastry. Bake for 40 to 45 minutes until pastry is golden brown and beef registers 125 degrees F. Remove from oven and rest before cutting into thick slices.
NOTE: This can be made with a pork tenderloin as well. Follow the recipe exactly the same, just use pork instead of beef. And cook until internal temp is 140, aiming for 145 after resting.