Cut the beef tenderloin into 1-inch pieces, season with salt and black pepper, then sear in hot olive oil until browned.

Sauté minced onion and chopped mushrooms in skillet until softened, then add soy sauce, dried oregano, and olives.

Roll out puff pastry and cut into squares, keeping them cold.

Spread mustard on pastry squares, add mushroom mixture and pieces of beef, then fold and seal the edges.

Arrange on a baking sheet, brush with beaten egg, and bake at 400°F (200°C) until golden brown.

Let cool slightly before serving to allow flavors to meld and enjoy!