Peel, wash and chop the potatoes. Cook in salted water for about 20 minutes. Peel onions and cut into fine rings. Fry the bacon in a pan without fat until crispy, remove. Fry onions in hot bacon fat for about 5 minutes.
Sprinkle with sugar and caramelize. Deglaze with vinegar and stir in 1 tablespoon butter, bring to the boil briefly. Season with salt and pepper. Remove from the stove.
Wash and halve the tomatoes. Heat oil in a pan. Sauté the tomatoes well in it. Season with salt and pepper.
Drain the potatoes. Add milk and 3 tablespoons of butter and mash everything coarsely. Season the puree with salt and nutmeg. Arrange mashed potatoes with bacon, onions and tomatoes.