1. 1. Prepare the Beef: Season the beef tenderloin cubes with salt and pepper. Heat a large skillet over high heat and sear the beef on all sides until a nice crust forms, about 2 minutes per side. Transfer the seared beef to a plate and let it cool slightly.

2. 2. Make the Mushroom Duxelles: In the same skillet, melt the butter over medium heat. Add the chopped mushrooms and garlic, and sauté until the mushrooms are softened and any excess moisture has evaporated, about 5-7 minutes. Season with salt and pepper, then let the mixture cool completely.

3. 3. Assemble the Mini Wellingtons: Unfold the puff pastry sheet and cut it into 12 equal squares. Place a small spoonful of the mushroom duxelles onto the center of each pastry square. Top each with a cube of seared beef and a small amount of chopped prosciutto. Brush the edges of the pastry with the egg wash.

4. 4. Bake the Mini Wellingtons: Preheat your oven to 400°F. Carefully transfer the assembled mini Wellingtons to a baking sheet lined with parchment paper. Bake for 15-18 minutes, or until the pastry is golden brown and flaky. Allow the Mini Beef Wellington Bites to cool for a few minutes before serving.