Sear beef in hot oil 8–10 minutes; brush with mustard, cool.
Pulse mushrooms in a processor. Sauté shallots, garlic in butter 2–3 minutes; add mushrooms, thyme, cook 8–10 minutes. Add wine; cook until dry. Season, taste, and adjust the seasoning. Cool.
Wrap beef in prosciutto and duxelles using plastic wrap; chill 20 minutes.
Wrap in puff pastry, seal with egg wash; chill 15 minutes.
Preheat oven to 400°F; brush pastry with egg wash, bake 35–45 minutes until 135°F. Rest 10 minutes.
Slice, serve on a white ceramic platter with thyme, potatoes, and wine.
Notes
Use a thermometer for doneness.
Store in the fridge for 3 days or freeze unbaked for 2 months.