Wash potatoes thoroughly and cook in boiling water for about 20 minutes. Drain the potatoes, rinse with cold water, peel and let them cool down. Peel and chop onion. Mix stock, vinegar, onion, salt, pepper and sugar, bring to the boil and add 4 tablespoons of oil.
Peel and slice the potatoes. Pour marinade over them and let them stand for at least 30 minutes. Clean, wash and spin-dry lamb's lettuce. Wash the meat, dab dry and season with salt and pepper.
Whisk the egg in a deep dish. Put flour and breadcrumbs on a flat plate each. Turn the cutlets first in flour, then in egg and finally in breadcrumbs. Heat 2 tablespoons of oil and fry 4 escalopes in it for about 4 minutes on each side until golden brown.
Fry the remaining schnitzel in the same way. Season potato salad to taste and carefully fold in lamb's lettuce. Arrange salad and schnitzel on plates and garnish with lemon.