Clean, wash and drain the spinach. Peel and finely dice the onion and sauté in 2 tablespoons of hot oil. Add the spinach and let it collapse. Season spinach with salt and pepper.
Wash, clean and slice the tomatoes. Stone the olives. Peel garlic. Wash and dry lemon. Peel a piece of lemon peel thinly and cut into narrow strips. Grate the rest of the peel, then squeeze the lemon.
Wash the basil, dab dry and cut into strips, keeping a few leaves for garnishing. Roughly dice toast. Coarsely chop together with garlic and olives in the universal chopper.
Fold in grated lemon peel and basil. Season with salt and pepper. Fry until crispy in 2 tablespoons of hot oil. Wash the fish fillet, dab dry. Sprinkle with lemon juice, season with salt and pepper.
Line the bottom of a casserole dish with slices of tomato, add spinach with cooking liquid. Place the fish on top. Spread olive and toast bread mixture on top. Bake in a preheated oven (electric: 200 °C/ gas: level 3) for about 20 minutes.
Sprinkle with olive oil in between. Arrange on plates and garnish with basil and lemon peel strips.