Wash the tomatoes, cut them in half or leave them whole, depending on size. Peel onions, cut into thick rings. Wash the basil, pluck the leaves and put some aside.
Wash the fillets, dab dry and season with salt and pepper. Heat the oil in a frying pan. Fry the fillets for 2 minutes on each side. Take them out.
Sauté onions in hot frying fat. Steam the tomatoes briefly. Season with some sugar, salt and pepper. Add approx. 200 ml water and stock, bring to the boil. Put the fillets in and stew covered for about 10 minutes.
Mix in basil and lemon peel. Season to taste and garnish with the rest of the basil.