Peel, wash and quarter the potatoes. Cook in boiling salted water for about 20 minutes. Peel and finely dice onions. Heat oil in a pan and sauté onions. Fold in pesto.
Wash the fish, dab dry. Season with salt and pepper. Wash, clean and slice the tomatoes. Distribute tomatoes in 2 casseroles. Place fish on top and spread with onion pesto.
Sprinkle with basil leaves. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 15-20 minutes. Drain potatoes, add buttermilk, mash finely and season with salt.
Serve the fish au gratin with mashed potatoes. Serve with a mixed cucumber and radish lettuce salad.