Cut sinew from veal, place veal in bowl, pour over combined wine and sherry, leave to stand 4 hours, turning occasionally. Remove veal and pat dry; reserve marinade. Heat oil and butter in pan, add veal and brown on all sides; remove and place in ovenproof dish. Pour off pan juices, leaving 1 tbsp in pan. Add peeled and chopped onion, cook 2 minutes.

Add flour, stir 1 minute, add strained marinade, cream, crumbled stock cube, water, worcestershire sauce, thyme and tarragon, stir until boiling. Pour sauce over veal, cover, bake in moderate oven 1½ hours or until tender.

Melt extra butter in separate pan, add thinly sliced mushrooms, cook over high heat 2 minutes, remove from pan. Remove veal from dish and keep warm. Pour sauce into saucepan, add mushrooms; combine cornflour and extra water, add to pan, stir until boiling; season with salt and pepper. Pour over sliced veal.