Preheat your oven to 400°F (200°C).
In a skillet, heat olive oil over medium heat. Sauté onions and garlic for about 5 minutes until soft. Add chopped mushrooms; cook until moisture evaporates.
In a bowl, combine the sautéed veggies with ground beef, salt, pepper, and Dijon mustard until well mixed.
Roll out the puff pastry on a floured surface into a rectangle.
Spread Dijon mustard on the pastry. Place the beef mixture in the center and fold over edges to secure.
Transfer to a parchment-lined baking sheet and brush with beaten egg.
Bake for 25-30 minutes or until golden brown. Let rest for 10 minutes before slicing.
