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 is quite soft. Lift the vegetables out of the pan and keep warm, leaving the flavoured butter in the pan to fry the veal. Drain the veal escalopes from the marinade, allowing surplus marinade to drain from the meat. Add to the hot butter and fry gently for about 4–5 minutes on each side. Lift the veal from the pan on to a hot serving plate and keep warm.
Measure the sherry into the frying pan and heat briskly, stirring until almost evaporated. Replace the cooked onion and mushroom in the pan, season with salt and pepper and stir in the cream. Stir until almost boiling, then draw the pan off the heat. Pour the sauce over the veal and serve.