Start by heating 1 tablespoon of olive oil in a skillet over medium-high heat. Season 1 pound of beef cubes (tenderloin or filet mignon) with salt and pepper, then sear them in the hot skillet for 2-3 minutes on each side until browned but not fully cooked. Remove the beef and let it cool on a plate while you prepare the mushroom duxelles.

In the same skillet, add the finely chopped mushrooms, 2 minced shallots, and 2 cloves of minced garlic. Cook over medium heat for about 5-7 minutes, stirring occasionally, until the moisture has evaporated and the mushrooms are golden brown. Stir in 1 teaspoon of thyme, season with salt and pepper, then remove from heat and allow the mixture to cool.

On a floured surface, roll out 1 sheet of store-bought puff pastry to about 1/8 inch thick. Use a sharp knife or pizza cutter to cut the pastry into 3x3 inch squares. Gather the scraps, roll them out again, and cut additional squares as needed. Set the pastry squares aside on a parchment-lined baking sheet.

Place a spoonful of the cooled mushroom duxelles in the center of each pastry square. Top each with a piece of the seared beef, and optionally brush a thin layer of Dijon mustard on top of the beef for added flavor. Carefully fold the corners of the pastry over the filling, and seal the edges using a fork to ensure there are no air pockets.

Preheat your oven to 400°F (200°C). Arrange the assembled Mini Beef Wellington Bites on a baking sheet lined with parchment paper, brushing each pastry with a beaten egg for a shiny, golden finish. Bake in the preheated oven for 15-20 minutes, or until the pastries are puffed and golden brown, signaling that they are perfectly cooked.

Once baked, remove the Mini Beef Wellington Bites from the oven and let them cool slightly on a cooling rack. They are best served warm, so indulge your guests with these gourmet appetizers while they are at their freshest and most delicious!