Prepare the filling
Heat olive oil in a pan over medium heat. Add minced garlic and cook for about 30 seconds. Add the chopped spinach and cook until wilted, about 2–3 minutes. Remove from heat and let it cool. Mix in feta, mozzarella, and cream cheese. Season with salt and black pepper.
Prepare the pastry
Roll out the puff pastry sheet on a flat surface. Cut into equal squares or rectangles.
Add the filling
Spread or place a portion of the spinach and cheese mixture onto each piece of pastry.
Shape
Fold the pastry to form triangles. Press the edges firmly to seal.
Egg wash and topping
Brush each triangle with beaten egg. Sprinkle sesame or nigella seeds on top if using.
Bake
Preheat oven to 180°C (350°F). Place the triangles on a lined baking tray and bake for 20–25 minutes, or until golden brown and crisp.