Peel onions and cut them into fine rings. Wash potatoes thoroughly and cut thinly across without cutting through the potatoes. (Works very well if you put the potatoes between 2 flat stirring spoon stems). Put onion rings into some potato incisions
Knead butter and 1/2 teaspoon salt. Put potatoes on a fat pan, cover with butter flakes and bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for approx. 50 minutes. Grate cheese. Sprinkle the potatoes with the cheese and bake in the oven at the same temperature for about 10 minutes or until the potatoes are soft
In the meantime wash and drain the salad. Wash the tomatoes, dab dry and cut in half. Season vinegar with salt, pepper and sugar, fold in oil. Mix salad, tomatoes and vinaigrette, season again. Take the potatoes out of the oven, arrange them on plates with the salad