1. Heat butter and oil in a large heavy-based frying pan. Remove the pan from the heat and put in layers of potatoes, onions and grated Cheese ending with a top layer of Cheese. 2. Cover and cook the vegetables gently for about 30 minutes or until the potatoes and onions are almost cooked. 3. Uncover and brown the top of the dish under a hot grill. Serve straight from the pan.