Finely crumble the ciabatta in the universal chopper. Thoroughly peel, wash and cut the celeriac into 8 slices. Blanch in boiling salted water for about 2 minutes. Remove, rinse with cold water and drain.
Drain the mozzarella and cut into 4 slices. Place 1 slice of mozzarella and 1 slice of salami on each slice of celery, cover with a second slice of celery. Whisk the eggs in a deep plate.
Put the crumbs in a second plate. Carefully roll the schnitzel first in egg, then in the crumbs. Press the crumbs well to the edge. Peel, wash and chop the potatoes. Cook in boiling salted water for about 20 minutes.
Peel and finely chop the onion. Heat 1 tablespoon of oil in a saucepan and sauté the onion. Add tomato paste and herbs, sweat briefly. Add tomatoes and 150 ml water. Bring to the boil, stir in broth, simmer over low heat for about 10 minutes.
Heat 2 tablespoons of oil and 1 tablespoon of fat in a frying pan. Fry the escalopes in this pan over a low heat for 5-6 minutes, turning them over. Heat milk and 1 tablespoon of fat. Drain the potatoes and let them evaporate briefly.
Pour in the milk mixture, mash everything to puree. Season to taste with salt and nutmeg. Season sauce with salt, pepper and sugar. Serve schnitzel with puree and sauce. Dribble frying fat over the puree.
Garnish with basil.