In a large skillet, heat a drizzle of olive oil over medium-high heat. Season the beef cubes generously with salt and pepper, and carefully place them in the hot skillet. Sear each side for about 2-3 minutes until a rich brown crust forms. Remove the beef and let it cool on a plate.

Using the same skillet, add a touch more olive oil if necessary and reduce the heat to medium. Sauté the finely chopped mushrooms, shallots, and minced garlic for 5-7 minutes until the mixture becomes golden and fragrant. Add fresh thyme and season with salt and pepper, stirring continuously. Set aside to cool once the duxelles reach a deep, caramelized color.

On a lightly floured surface, roll out the chilled puff pastry to a thickness of about 1/8 inch. Use a sharp knife to cut the pastry into 3x3 inch squares. Keep the dough chilled until you’re ready to assemble the Mini Beef Wellington Bites.

Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper. Take a pastry square, place a spoonful of the cooled mushroom duxelles in the center, followed by a beef cube. If desired, spread a thin layer of Dijon mustard on the beef for added flavor. Fold the pastry over the filling and pinch or press the edges with a fork to seal tightly.

Place the assembled Mini Beef Wellington Bites on the prepared baking sheet, leaving space between each one. Brush the tops generously with beaten egg. Bake in the preheated oven for 15-20 minutes, or until the pastry is puffed and golden brown.

After baking, remove the Mini Beef Wellington Bites from the oven and let them cool slightly on a wire rack. Serve warm with your favorite dipping sauce.