Preheat the oven to 400°F (200°C) and line a baking sheet with parchment paper.
 Roll out the puff pastry slightly and cut it into 2–4 squares depending on the desired size.
 Using a knife, lightly score a smaller square about 1 cm inside the edges to create a border (do not cut all the way through).
 Beat the egg with the milk and brush it over the pastry edges for a golden crust.
 Sprinkle the shredded mozzarella and Gruyère evenly in the center area of each pastry square.
 Drizzle lightly with olive oil and season with black pepper and a pinch of salt.
 Bake for 15–18 minutes until the pastry is puffed, golden, and the cheese is melted and bubbly.
 Remove from the oven and sprinkle fresh rosemary on top. Serve warm.