1. Preheat the oven to 400°F (200°C) and line a baking tray with parchment paper.

2. Heat olive oil in a pan over medium heat. Sauté the minced garlic for about 2 minutes until soft.

3. Add the baby spinach to the pan, season with salt and pepper, and sauté until wilted. Remove from heat and allow to cool.

4. Cut the puff pastry sheet into 6 or 12 squares, depending on the desired size, and place them on the baking tray.

5. In a mixing bowl, combine the cooled spinach, cream cheese, mozzarella cheese, and one beaten egg. Season with salt and pepper, then mix thoroughly.

6. Divide the spinach mixture evenly among the pastry squares. Fold two opposite edges of each square, pressing them tightly together.

7. Brush the tops of the spinach puffs with egg yolk.

8. Bake for 25 to 35 minutes, or until the puffs are golden brown.