Sear the beef: Season beef tenderloin generously with salt and pepper. Heat olive oil in large skillet over high heat until smoking. Sear beef on all sides for 2-3 minutes total until deeply browned. Transfer to plate and refrigerate until completely cool, about 30 minutes. This prevents pastry from getting soggy.

Make mushroom duxelles: Pulse chopped mushrooms in food processor until very finely minced (almost paste-like). Melt butter in large skillet over medium-high heat. Add shallots and cook 2 minutes. Add mushrooms, garlic, and thyme. Cook, stirring frequently, for 10-12 minutes until all moisture has evaporated and mixture is dry and paste-like. Add sherry and cook until evaporated. Season with salt and pepper. Cool completely.

Assemble prosciutto layer: Lay large sheet of plastic wrap on work surface. Arrange prosciutto slices in overlapping rectangle, about 12×10 inches. Spread cooled mushroom duxelles evenly over prosciutto. Spread pâté thinly over cooled beef. Place beef at bottom edge of prosciutto rectangle.

Roll and chill: Using plastic wrap to help, tightly roll prosciutto and mushrooms around beef, creating tight cylinder. Twist ends of plastic wrap to seal and refrigerate for 30 minutes to firm up.

Wrap in pastry: Roll puff pastry into rectangle about 14×12 inches and 1/4 inch thick. Remove plastic wrap from beef. Place beef seam-side down in center of pastry. Brush edges of pastry with egg wash. Fold pastry over beef, trimming excess and pressing seams to seal tightly. Tuck ends under. Place seam-side down on parchment-lined baking sheet. Refrigerate 15 minutes.

Bake: Preheat oven to 425°F. Brush entire pastry with egg wash. Score decorative pattern on top with knife (don’t cut through). Sprinkle with flaky sea salt. Bake for 35-40 minutes until pastry is deep golden brown and internal temperature reaches 125°F for medium-rare (130°F for medium). Let rest 10 minutes before slicing into thick portions.