Finely chop mushrooms in a food processor. Sauté shallots and garlic in butter until soft. Add mushrooms and thyme; cook over medium heat until all liquid evaporates and mixture is dry (duxelles). Cool completely.

Season beef cubes well with salt and pepper. Sear quickly in hot oil/butter (20 seconds per side) just to brown. Remove and cool. Brush seared beef with Dijon mustard.

Preheat oven to 400°F (200°C). Roll out puff pastry and cut into squares large enough to wrap beef.

Place a teaspoon of duxelles on a pastry square, top with a beef cube, and wrap tightly, pinching seams to seal.

Place seam-side down on a lined baking sheet. Brush with egg wash, score tops lightly, and sprinkle with coarse salt.

Bake for 15-18 minutes until pastry is golden brown. Let rest 5 minutes before garnishing with thyme and serving.