Step 1: Prepare the Dough

1. In a large mixing bowl, combine flour and salt. Add the chilled butter and use your hands or a pastry cutter to work it into the flour until the mixture resembles coarse crumbs.
2. Stir in the Greek yogurt and egg yolk, mixing until the dough starts to come together.
3. Gradually add cold water, one tablespoon at a time, until the dough forms a firm but pliable ball.
4. Wrap the dough in plastic wrap and chill in the refrigerator for at least 30 minutes.

Step 2: Make the Spinach Filling

1. Heat olive oil in a skillet over medium heat. Add the chopped onion and cook for 3-4 minutes, until soft and translucent.
2. Stir in the garlic and cook for 30 seconds, just until fragrant.
3. Add the fresh spinach, stirring until wilted (about 2 minutes). If using frozen spinach, make sure to drain excess water before adding it to the pan.
4. Season with salt, black pepper, oregano, and red pepper flakes (if using). Remove from heat and let the mixture cool for a few minutes.
5. In a mixing bowl, combine the spinach mixture, ricotta, feta, and beaten egg. Stir until fully combined.

Step 3: Assemble the Pie

1. Preheat your oven to 375°F (190°C).
2. Roll out the chilled dough on a floured surface into two equal-sized circles (about 10 inches each).
3. Place one dough round on a parchment-lined baking sheet. Spread the spinach filling evenly over it, leaving a ½-inch border around the edges.
4. Place the second dough round on top and press the edges together to seal. Use a fork to crimp the edges for a decorative touch.
5. Brush the top of the pie with egg wash for a golden crust. Use a sharp knife to cut a few small slits in the top to allow steam to escape.

Step 4: Bake the Pie

1. Bake in the preheated oven for 30-35 minutes, until the crust is golden brown and crisp.
2. Let the pie cool for 10 minutes before slicing. This helps the filling set and makes it easier to cut.

Step 5: Serve and Enjoy!

Slice into wedges and serve warm or at room temperature. Enjoy it as a main dish, side, or snack!