Cut the meat in 1½-in. cubes, trimming off most of the fat. Place in a bowl with the sliced onion, seasoning, lemon juice and wine or cider. Mix well together and leave to stand overnight. When ready to cook, arrange the meat on skewers, alternating pieces of meat with a whole tomato or a mushroom. Brush with butter or dripping and grill for 15 min., turning frequently. Serve on the skewers with vegetables and potatoes or savoury rice.