Peel, wash and cut the potatoes into large pieces. Cover and cook in about 2 l boiling salted water for about 20 minutes. Peel onions and cut into very fine rings. Cut bacon into small cubes.
Fry the bacon in a pan without fat until crispy. Take out and drain on kitchen paper. Steam onions in the frying fat until transparent. Sprinkle with sugar and caramelise while stirring.
Deglaze with vinegar. Add 1 tablespoon butter and simmer for about 2 minutes. Remove from heat. Season with salt and pepper.
Wash, dry and halve the tomatoes. Heat the oil in a pan. Sauté the tomatoes in it. Season with salt and pepper. Remove the pan from the stove. Stir milk and 3 tbsp. butter in a small pan.
Drain the potatoes. Pour in warm milk mixture. Mash potatoes with a potato masher until the liquid is completely mixed. Season to taste with salt and nutmeg. Arrange mashed potatoes with bacon, onions and tomatoes.