Peel and wash the potatoes and celery and cut into large cubes. Cook the potato and celery cubes in boiling salted water at medium heat for about 15 minutes until soft. In the meantime, clean salads, pluck off leaves, wash and spin dry.
Wash the cherry tomatoes, cut them in half and put them in a bowl. Mix vinegar, 1 teaspoon mustard, salt and sugar. Gradually add 4 tablespoons of oil. Peel onions and cut 4 into rings, dice the rest finely.
Put mince in a bowl. Add the egg, 2 teaspoons of mustard, diced onion and breadcrumbs, season with salt and pepper and knead well. Pour the potato-celery into a sieve, put it back into the pot and let it evaporate at medium heat.
Put 150 ml milk in a pot, season with salt, nutmeg and pepper and bring to the boil once. Add potatoes and celery and mash to puree. Remove from heat and cover. Form 8 meatballs from the minced meat.
Heat a pan with 2 tablespoons of oil and fry the meatballs over medium heat until golden brown. Remove the meatballs from the pan. Fry the onion rings in the pan until golden brown. Season with salt, pepper and a pinch of sugar.
Add cream and 100 ml milk and bring to the boil once. Add lettuce to the tomatoes and mix with the dressing. Arrange 2 meatballs each with sauce, mashed potatoes and celery and salad on plates.