Preheat the oven to 200°C. Butter a deep, 6–7 cup baking dish and arrange the onion slices in layers on the bottom.
Cut the cheeses into large, thin slices and arrange them, overlapping, on top of the onions to cover them completely.
Mix the butter with the chopped sage and drizzle over the cheese. Bake in the top of the oven for 30–40 minutes, or until the cheese has melted and lightly browned and the onions are cooked. (As the onions cook, the level of the food in the dish will drop slightly.)
Garnish with the fresh sage leaves and serve hot as an accompaniment to sausages, roasts or chops, or as a supper dish with a salad and wholemeal bread.