Spread out deep-frozen potato slices on a baking tray and bake in a preheated oven (electric cooker: 225 °C/ gas: level 4/ convection oven: 190-200 °C) for 18-22 minutes. In the meantime, peel onions and cut them into rings except for one.
Finely dice the remaining onion. Clean, wash and drain the salad well. Pluck the frisée into bite-sized pieces. Wash tomato, dab dry and cut into quarters. Remove seeds and finely dice the flesh.
Wash the herbs, dab dry and chop. Mix yoghurt, lemon juice, onion cubes, herbs and tomatoes. Season to taste with salt and pepper. Heat oil in a pan and fry the steaks for 2-3 minutes on each side.
Season with salt and pepper, remove from the pan and let it rest for about 5 minutes. In the meantime, add onions and butter to the hot frying fat and fry until golden brown. Put the salad in a bowl and spread the salad dressing over it.
Arrange steaks with roasted onions and skin-potatoes on plates. Add mayonnaise and ketchup and garnish with rosemary to taste. Serve with the salad.