Sift the flour, baking powder and salt together. Rub in the butter and add the grated cheese. Stir in the milk and using a fork mix to a rough dough in the basin. Turn out on to a floured surface and knead lightly until smooth. Cover and rest in a cool place for 10 minutes while preparing the topping.
Heat the oil in a saucepan. Add the onion and fry gently for a few moments until softe