Preheat the oven to 180 °.

Clean the onions and cut them into slices of about ¾ cm.

Beat the egg in a bowl.

Cut the mozzarella into cubes and place it in the bowl with the egg. Season with salt and pepper.

Roll out the dough with its parchment paper on a baking sheet. With a knife lightly trace an edge of about 2 cm and brush with a little oil. Prick the center with a fork.

Pour the mixture into the center and cover with the onion rings. Season with salt, pepper and a drizzle of oil.

Bake the salted cake in the oven for about 30/35 minutes or until it is golden brown and the onions cooked.

Serve it hot or at room temperature.