For the sauce, peel and halve the onions and cut into cubes or strips. Heat the clarified butter in a roasting pan and fry the onions until golden, stirring constantly.
Pull the pot to one side, sprinkle the flour over the onions and stir with a mixing spoon. Put the saucepan back on the hob and brown the roux, stirring constantly, until light brown.
Pull the pot to the side again, deglaze the roux with the stock, bring to the boil once. Add the soy sauce and white wine to the sauce.
Cover the onion sauce and let it slowly simmer for about 10 minutes. Season with salt, pepper and lemon juice. If the onion sauce has become too thick, dilute it with a little water.
Beat the turkey schnitzel a little and season with salt and pepper. Put the egg in a deep plate and beat with a little milk. Place breadcrumbs on a flat plate. Turn the seasoned turkey schnitzel first in egg, then in breadcrumbs and press down a little. Heat the fat, fry the schnitzel in it for 4 minutes on each side. Serve the hot sauce with the schnitzel.