Clean, wash and slice the zucchini. Peel and finely chop the onion. Heat 1 tablespoon of oil in a frying pan, fry the zucchini and onion while turning them, season with salt and pepper and deglaze with white wine.
Pour into a casserole dish. Wash the fish, pat dry and season with salt and pepper. Fold the tail ends down a little. Place the fish on the zucchini in the baking dish and sprinkle with 1 tablespoon of oil.
Cook in the preheated oven (electric cooker: 150 °C/ convection oven: 125 °C/ gas: level 1) for approx. 20 minutes. In the meantime, peel and wash the potatoes and cook them in boiling salted water for about 20 minutes. Wash parsley, shake dry and chop finely, except for a little bit for garnishing.
Peel the garlic and cut into fine cubes. Wash the tomatoes, dab dry, cut into quarters, remove seeds. Cut tomatoes into small cubes. Wash lemon, grate dry and peel with a zest ripper.
Halve the lemon, squeeze the juice from one half. Mix prepared ingredients and 2 tablespoons of oil and season with salt and pepper. Take the fish out of the oven, spread Gremolata on the fish. Drain the potatoes and add them.
Garnish with parsley.