Clean and thinly slice the leeks.

Melt the butter in a large skillet, add the leeks, and cook until softened. Pour in 1/2 cup water and add the bouillon cube.

Mix the sour cream and cheese into the leeks. Season with salt and pepper.

Line a 9-inch (24 cm) pie pan with one pie crust, keeping the parchment paper underneath. Prick the bottom with a fork, then spread the leek mixture evenly. Cover with the second pie crust.

Press the edges of the two crusts together and trim the excess, leaving a 1/2-inch (1 cm) border. Gently roll the edge and press with a fork to seal.

Brush the top with the egg yolk mixed with 1 tablespoon water.

Bake for 30 minutes.