Wash potatoes thoroughly and boil in water for about 20 minutes. In the meantime peel and finely grate the onion. Knead minced meat, tomato paste, mustard, 1 egg, onion and breadcrumbs. Season with salt and pepper.
Form small meatballs from the mass. Heat the oil in a pan. Fry the meatballs in it in 2 portions for about 5 minutes. Drain the potatoes and rinse briefly in cold water, peel off the skin.
Wash the tomatoes. Whisk cream, créme fraiche and 4 eggs well. Season with salt, pepper and nutmeg. Grease an ovenproof dish (18 x 22 cm). Layer potatoes, tomatoes and meatballs. Pour egg mixture over it.
Cover with aluminium foil and bake in a preheated oven (electric range: 200 °C/ convection oven: 175 °C/ gas: level 3) for approx. 30 minutes. Sprinkle the casserole with cheese and bake for another 10 minutes. In the meantime, wash the chives, dab dry and cut into small rolls, except for a few stalks for garnishing.
Sprinkle the casserole with chives and garnish.