Prepare the puff pastry: Thaw the puff pastry sheets according to package instructions. Spray a 24-count mini muffin pan with nonstick cooking spray.

Roll out the pastry: On parchment paper, roll out the puff pastry sheets to about 12 inches square. Using a 2 ½-inch round cutter, cut 24 circles and press them into the muffin pan.

Brush with mustard: Brush the inside of each pastry cup with Dijon mustard. Cover and refrigerate while you prepare the filling.

Preheat oven: Preheat the oven to 425°F (220°C).

Prepare the mushroom filling: Pulse the mushrooms in a food processor until finely minced. In a skillet, melt 2 Tablespoons butter, then sauté the mushrooms until golden brown. Add prosciutto, garlic, thyme, and vermouth. Cook until the vermouth has reduced and the prosciutto is softened. Season with ½ teaspoon salt and ¼ teaspoon pepper, then stir in the remaining butter and let cool.

Prepare beef: Season the beef tenderloin with the remaining salt and pepper.

Assemble: Remove the muffin pan from the refrigerator. Add a few pieces of beef to each pastry cup, then top with a teaspoon of the mushroom mixture.

Bake: Bake for 12 minutes or until the pastry is golden brown and the beef is cooked to your liking.

Make horseradish cream sauce: Mix sour cream, mayonnaise, horseradish, and honey in a small bowl.

Serve: Remove from the oven and serve with the horseradish cream sauce on the side.