Peel and wash the potatoes and cut them into thin slices. Zucchini likewise wash, clean and cut into thin slices. Layer the vegetables alternately in flakes in a large gratin dish.
Cut the tomatoes into strips. Peel and finely chop the onions. Mix half of the onions and tomato strips with cream, season with salt and pepper. Pour over the gratin. Grate the cheese and sprinkle over the gratin.
Cook in the preheated oven (electric cooker: 200 °C/ gas: level 3) for about 45 minutes. Fry the rest of the onions and lentils in 1 tablespoon of oil. Deglaze with 100 ml water, season. Cover and cook for about 10 minutes.
In the meantime clean and wash the salads and pluck them into bite-sized pieces. Add the rest of the oil and vinegar to the lentils. Season again and spread over the plucked salad. Garnish with parsley.