• Thaw puff pastry in the fridge overnight. Then let it sit at room temperature for 45 minutes.

Prepare The Spanakopita Filling:

• Melt butter in a saute pan.
• Saute the spring onions until soft.
• Add the spinach in batches. Season lightly with salt and wait until the first batch of spinach wilts before adding the next one. Cook over medium-high heat.
• Stir in the flour with a wooden spoon. Turn the heat to medium-low.
• Pour in the milk and cook, stirring frequently, until it becomes creamy and thickens. Take the pan off the heat.
• Let the filling cool for at least 30 minutes. It should be warm, not hot, before adding the eggs and cheese. Season with salt and pepper.
• Stir in the eggs, feta cheese, and dill.

Assemble:

• Preheat the oven to 175°C / 350°F.
• Prepare the pan. Brush a 23 x 33 cm (9 x 13 inches) pan or baking dish of a similar size with olive oil.
• Lay one puff pastry sheet in the pan and press gently to fit.
• Add the filling and spread it evenly with a spoon.
• Top with another puff pastry sheet. Press the edges of both sheets to stick together and create a pie edge. Trim it slightly wherever necessary.
• Divide into pieces. Cut and separate the pie into 15 square pieces without cutting all the way through to the bottom. Just the top puff pastry sheet.
• Whisk egg yolk with 2 tablespoons of water. Then brush the top of the pie.
• Bake for about 1 hour, or until the pie gets a nice honey-golden color. If you see it becoming puffy while it bakes, run a knife through the cuttings you made on the surface (they may stick together again) to allow the steam to escape.
• Cool for at least one hour at room temperature before cutting and serving.