Yield: 6-8 servings
Heat a large saute pan over medium-high heat. Season tenderloin, generously, with salt and pepper. Melt 2 tablespoons of butter in the pan and sear meat on all sides until well-browned. Remove meat from pan and brush with mustard. Set aside.
Return the same pan to medium heat and add remaining butter, shallots and garlic. Cook aromatics for a minute before adding mushrooms. Cook mushrooms stirring occasionally until all the water has cooked out and mushrooms form a paste (about 15 minutes). Remove from heat while adding cognac. Return to heat and evaporate cognac before adding cream. Add truffle salt and continue cooking until the mixture has formed a thick paste. Adjust seasoning with sea salt and ground pepper. Cool mushrooms completely.
Unroll the phyllo sheet and lay flat on a cutting board. Line phyllo with prosciutto, overlapping the slices. Spoon cooled mushroom mixture over prosciutto into a thick layer about 4 inches longer than the meat.
Position tenderloin at the edge of mushrooms. Roll phyllo and meat to completely wrap meat (kind of like rolling sushi!).
Lightly flour a work surface and roll puff-pastry to about ¼ inch thickness. Place tenderloin at the edge of the puff pastry and roll, being sure to cover tenderloin. Cut excess puff pastry. Fold over and seal edges so the meat is completely covered, (Like wrapping a gift) brush with lightly beaten egg. Reserve beaten egg and store in the fridge.
Roll the entire package tightly in parchment paper and then in foil. Refrigerate until completely chilled and firm. The Wellington can be stored in the refrigerator for up to 2 days before serving.
Preheat the oven to 425F. Unwrap wellington and place on a parchment lined baking sheet. Brush with remaining lightly beaten egg and sprinkle with sea salt. Lightly score puff pastry in a decorative pattern. Bake for 35-45 minutes or until a thermometer registers 120F for medium-rare.
Allow to rest for 10 minutes before cutting.