Wash the potatoes and cook in boiling salted water for about 20 minutes. Wash the fish fillet, dab dry, sprinkle with lemon juice, salt and cover and leave to stand for about 20 minutes.
Wash the tomatoes and remove the stalk. Cut the tomatoes into slices. Wash the herbs and chop them coarsely, except for some thyme and parsley for garnishing. Peel and finely chop onion and garlic.
Knead butter, chopped ingredients, breadcrumbs, half of the Parmesan cheese and lemon zest. Season the mixture with salt and pepper. Rinse potatoes cold, peel and cut into thin slices. Layer the potato and tomato slices in a greased casserole dish (approx. 30 cm long).
Place the slices one layer higher on the edge. Mix cream and remaining Parmesan, season to taste and pour over the slices. Place the fish fillet in the middle of the dish. Spread the herb mixture over it and press it down a little.
Bake the casserole in a preheated oven (electric oven: 200°C/ gas: level 3) for approx. 25 minutes. Sprinkle with the remaining thyme. Serve garnished with the remaining parsley and lemon slice.
Served with a colourful fresh salad with sour cream and herb dressing.