Prepare the Beef: Season and sear the beef tenderloin pieces until they're browned on all sides, then set them aside to cool.
Mushroom Filling: Sauté onions and mushrooms, then blend them with olives and seasonings to create a flavorful paste.
Assemble: Cut the puff pastry into squares, add mustard, the mushroom mixture, and a piece of beef to each. Fold the pastry over the filling and seal.
Bake: Place the bites on a baking tray, brush with egg wash, and bake until golden and puffed.
Serve: Enjoy the Beef Wellington bites warm, paired with your favorite dipping sauce.