Preheat oven to 400°F and spray a 12-cup muffin tin with cooking spray.
Heat butter in a large skillet over high heat, season beef cubes with salt and pepper, and sear for 2 minutes until browned on all sides, then remove and set aside.
In the same skillet, add mushrooms and garlic, cooking over medium-high heat for 6-8 minutes until all moisture evaporates and mushrooms are deeply browned, then stir in thyme and heavy cream.
Cut puff pastry into 12 squares and press each into a muffin cup, then brush the bottom of each with Dijon mustard.
Divide the mushroom mixture and beef cubes among the pastry cups, fold edges slightly over the filling, and bake for 12-15 minutes until pastry is golden brown and puffed.