1-First Step: Pat 12 beef medallions dry with paper towels. Season all sides with salt and pepper. Let sit 10 minutes at room temp for even cooking. Chop mushrooms fine in food processor pulses. Mince shallot, garlic, thyme. Thaw puff pastry in fridge overnight or counter 40 minutes. Preheat oven to 425°F. Line two baking sheets with parchment.

2-Second Step: Heat cast-iron skillet over high heat with 1 tsp oil. Sear beef 1 minute per side for crust, keeping center cool. Aim for rare stage now; it finishes baking. Transfer to plate, chill 15 minutes in freezer to firm. Wipe skillet clean.

3-Third Step: Melt 2 tbsp butter in skillet over medium heat. Add shallot, garlic; sauté 2 minutes until soft. Stir in mushrooms, thyme, pinch salt. Cook 5-7 minutes, stirring often, until moisture evaporates and mix drys like paste. No liquid pools. Cool 10 minutes. Taste, adjust pepper. This mushroom duxelles prevents soggy pastry.

4-Fourth Step: Unfold puff pastry on floured surface. Roll lightly to 1/8-inch thick. Cut into 4×4-inch squares (3 per sheet if standard). On each square center: 1-2 tsp duxelles, topped with prosciutto-wrapped beef (wrap beef first in 1-2 prosciutto slices). Brush pastry edges with egg wash. Fold corners over beef like envelope: bottom up, sides in, top down. Press seams tight, crimp with fork. Place seam-side down on sheet, 2 inches apart.

5-Fifth Step: Brush tops with egg wash for gloss. Score lightly with knife for vent. Chill assembled minis 30 minutes on tray. This sets pastry, prevents leaks. Bake 15-18 minutes until puffed, golden. Rotate sheets midway. Internal beef hits 135°F for medium-rare. Rest 5 minutes on rack.

6-Sixth Step: Serve warm with mustard, horseradish, or Dijon. Garnish parsley. Pair with salads or try our steak frites recipe for full meal. Cut in half if large. Leftovers store well (see storage section).