Preheat oven to 400°F; spray a 12-cup muffin tin.
Heat butter in a large skillet over high. Season beef with salt and pepper; sear 2 minutes until browned on all sides. Set aside.
Cook mushrooms and garlic in same skillet over medium-high 6-8 minutes until moisture evaporates and mushrooms are deeply browned. Stir in thyme and cream.
Cut pastry into 12 squares; press into muffin cups and brush bottoms with Dijon.
Divide mushrooms and beef among cups; fold edges slightly over filling. Bake 12-15 minutes until pastry is golden and puffed.