Prep & Sear Beef: Season tenderloin with salt and pepper. Sear in hot olive oil 1½–2 min per side until golden. Brush all over with Dijon mustard and cool.
Make Duxelles: Sauté shallots and garlic in butter, add mushrooms and thyme. Cook, stirring, until moisture evaporates and mixture is paste-like. Cool.
Wrap in Prosciutto: On plastic wrap, layer prosciutto slices into a rectangle. Spread duxelles (and pâté, if using) over prosciutto. Place beef in center, roll tightly, twist ends of plastic wrap, and chill 30 min.
Encase in Pastry: Roll pastry into a rectangle. Unwrap beef roll, place on pastry, brush edges with egg wash. Fold pastry over beef, seal seams, trim excess, add decorative pastry if desired. Chill 15 min.
Bake: Preheat oven to 400 °F (200 °C). Brush Wellington with egg wash. Bake on a parchment-lined sheet for 20–25 min (medium-rare) until internal temp is 125 °F (52 °C).
Rest & Slice: Rest under foil 10 min. Slice into 1–1½ inch rounds and serve.