Peel, wash and chop the potatoes. Cook in boiling salted water for about 20 minutes until soft. Peel onions and cut into fine rings. Cut bacon into small cubes.
Leave the bacon in a coated pan until crispy, take it out and keep it warm. Fry the onions in the frying fat at medium heat for about 5 minutes, sprinkle with sugar, caramelise, deglaze with vinegar.
Add 1 tablespoon butter and simmer for about 1 minute, 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. Heat milk and remaining butter in a pot. Drain the potatoes and let them evaporate briefly.
Add the milk mixture and crush everything coarsely. Season to taste with salt and nutmeg. Arrange mashed potatoes on plates, add bacon, onions and tomatoes.