Thaw the spinach leaves, drain and squeeze out lightly.
Peel the onion, dice it finely and sweat it in a non-stick pan. Add the spinach and sauté briefly. Wash, dice and add the tomatoes. Season well with salt, pepper and garlic and stir in the yogurt. Season the fish with salt and pepper.
Put about half of the spinach in a baking dish, place the fish on top and pour the rest of the spinach over it.
Cook everything in a preheated oven at 160 ° C for about 25 minutes. Serve immediately.