In a bowl, sift the flour, add salt and cut the butter into pieces.
Add the milk and egg yolk, then mix with your fingers until a dough forms. Wrap in plastic and chill for 15–20 minutes.
Heat a little olive oil in a pan and sauté the spinach for 2–3 minutes. Drain, add crème fraîche, season with salt and pepper, mix, and let cool. Then fold in the mozzarella pieces and set aside.
On a floured surface, roll out the dough. Using a cutter, make 12–14 cm discs.
Lightly brush the edges of each disc with water. Place a generous tablespoon of the spinach and mozzarella mixture in the center and fold to form half-moons.
Seal the edges with a fork and place on a baking tray lined with parchment paper.
Mix the egg yolk with milk and brush over the empanadas.
Bake for 15–20 minutes.
Serve as an appetizer or with a small salad for a meal!