Preheat the oven to 400°F (200°C).

In a skillet, heat olive oil over medium heat and sauté the mushrooms until they are browned and moisture has evaporated.

Add the beef to the skillet and cook until browned. Stir in Dijon mustard and remove from heat.

Roll out the puff pastry and cut into small squares.

Place a spoonful of the beef mixture in the center of each pastry square and fold over to seal.

Brush the tops with beaten egg and place on a baking sheet.

Bake in the preheated oven for 15-20 minutes or until golden brown.