Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). Mix butter and 1⁄2 tsp. salt. Peel and slice onions or cut them into fine rings. Wash potatoes thoroughly and cut them crosswise close together, but do not cut through.
Place the potatoes on a baking tray or in a large flat casserole dish. Put onion rings into some potato incisions. Cover potatoes with butter flakes. Bake in a hot oven for about 45 minutes.
In the meantime, sort out the salad, wash and drain. Wash and halve the tomatoes. Mix vinegar with salt, pepper and 1 pinch of sugar. Fold in the oil. Mix salad, tomatoes and vinaigrette.
Grate cheese. Sprinkle potatoes with it and bake in the oven at the same temperature for about 10 minutes. Serve the potatoes with the salad.