Trim the escalopes neatly and arrange in a shallow plate or dish. Mix together all the ingredients for the marinade and pour over the veal. Leave to marinate for about 30 minutes.
Meanwhile trim and slice the mushrooms for the sauce and peel and finely chop the onion. Melt the butter in a frying pan and add the mushrooms and onion. Fry over very gentle heat for 5 minutes until the onion