Preheat your oven to 400°F (200°C).
In a skillet over medium heat, add olive oil and sauté chopped onions until translucent. Then add sliced mushrooms and minced garlic. Cook until the mushrooms are tender. Season with salt and pepper.
Unfold the thawed puff pastry sheet onto a baking tray lined with parchment paper. Roll it out if needed to fit the tray.
Spread the sautéed mushroom mixture over half of the pastry sheet, leaving a border. Top with grated cheese and fold the other half over to create a pocket. Press down on the edges to seal.
Apply beaten egg (or almond milk) over the top of the pastry.
Bake for about 25–30 minutes or until golden brown and puffed up.
Let it cool slightly before slicing. Garnish with freshly chopped herbs.