1. Boil water and add one tablespoon of salt.
2. Add potatoes and cook for around 10 minutes or until you can stick a fork into it and it comes out easily.
3. Then drain.
4. Add spinach and mix until the spinach starts to wilt.
5. Then add mozzarella cheese, salt, and black pepper and mix until well combined. Set aside.
6. Preheat oven 400 degrees Fahrenheit.
7. To prepare puff pastry, sprinkle a little flour on the counter. Put puff pastry on the floured surface just to make it not stick to the counter. Cut each square into quarters.
8. Roll each square once just to make it easy to pinch and seal.
9. Add two tablespoons of the potato spinach mixture to the square.
10. Then take the two opposite sides of the square and put them together and seal the whole side to make a triangle.
11. Put on a baking sheet lined with aluminum or parchment paper.
12. Put on egg wash and sesame seeds if you want.
13. Bake for 20 minutes or until golden.