In a large skillet over medium heat, add the olive oil or butter. Once hot, add the minced garlic and sauté for about 30 seconds until fragrant.
Add the chopped spinach to the skillet, stirring to combine with the garlic. Cook for 2-3 minutes until the spinach is wilted. Remove from heat and let it cool slightly.
In a mixing bowl, combine the ricotta cheese, mozzarella cheese, grated Parmesan cheese, egg, black pepper, salt, and nutmeg. Stir in the sautéed spinach and garlic mixture until well combined.
Roll out the thawed puff pastry on a lightly floured surface to about 1/8 inch thick. Cut into squares, approximately 3 inches by 3 inches.
Place a heaping tablespoon of the spinach filling in the center of each pastry square. Fold the pastry over the filling to form a triangle, pressing the edges to seal tightly.
Transfer the prepared puffs to a baking sheet lined with parchment paper. Brush the tops with an egg wash made from one beaten egg mixed with a tablespoon of water.
Bake in the preheated oven for 15-20 minutes, or until golden brown and puffed up.
Once baked, let them cool slightly before serving. Enjoy warm!
Notes
For variations, consider adding different cheeses, herbs, or vegetables to the filling.