Cut the beef tenderloin into 1-inch cubes and season generously with salt and black pepper. Sear in heated skillet for 1-2 minutes.

In the same skillet, sauté minced onion and button mushrooms for about 10 minutes until softened and golden.

Add soy sauce, dried oregano, and more black pepper to the skillet. Blend the mixture in a food processor until smooth.

On a floured surface, roll out puff pastry and cut into 3 to 3.5 inch squares.

Preheat oven to 400°F (200°C). Spread mustard on each pastry square, add mushroom filling and a piece of seared beef. Seal the squares.

Arrange bites on a baking tray lined with parchment paper and brush the tops with beaten egg.

Bake for 10-15 minutes, until golden brown. Cool slightly before serving.