Peel and finely chop the onion. Heat 1 tablespoon of oil in a saucepan. Fry the onion in it. Add spinach and 75 ml water and heat for 7-9 minutes in a closed pot, season with salt and pepper.
Wash the meat, dab dry and pat flat. Season with salt and pepper. Heat 2 tablespoons of oil in a frying pan. Fry the meat for about 4 minutes on each side and remove. In the meantime, wash the tomatoes, grate them dry and cut them into fine slices.
Finely crumble the feta cheese. Peel garlic and cut into fine slices. Spread about 20 g spinach on each chicken fillet. Sprinkle tomato slices and feta cheese over it. Garnish with garlic slices.
Bake under the preheated grill until the cheese has melted slightly. Arrange on plates, add the rest of the spinach.