In a large mixing bowl, combine flour and salt, then cut in the cold butter until the mixture resembles coarse crumbs. Gradually add ice water, mixing just until combined to form a dough. Shape the dough into a disc, wrap it in plastic, and refrigerate for at least 30 minutes.

While the dough chills, heat olive oil in a skillet over medium heat. Add chopped onions and sauté until translucent, about 5 minutes. Stir in fresh spinach and cook until wilted, roughly 2-3 minutes. Remove from heat and mix in ricotta cheese, grated mozzarella, salt, and pepper until well combined.

Once the dough has chilled, preheat your oven to 375°F (190°C). On a lightly floured surface, roll out the pastry dough to about 1/8-inch thickness. Cut it into squares or circles, depending on your preferred shape.

Spoon a generous amount of the spinach and cheese filling into the center of each pastry piece. Fold the pastry over to create a pocket and press the edges securely with a fork to seal.

Brush the tops of the assembled pastries with beaten egg. Place the baking sheet in the preheated oven and bake for 20-25 minutes or until the pastries are puffed and golden brown.

Once baked, remove the pastries from the oven and let them cool on a wire rack for a few minutes. Serve warm as a delightful appetizer or snack.