1. Cook the chayotes in plenty of salted water until they are tender. Drain and set aside. In a bowl mix together the rice, cheese, milk, egg, salt and pepper to taste. In a saucepan heat 2 tablespoons of butter and fry the onion until tender.
2. Add the above mixture and cook, stirring constantly, until it has thickened. Add the drained chayotes, sugar and cook over low heat for 5 minutes.
3. Pour into a baking dish and sprinkle pieces of the remaining butte. Bake in a 425ยบ oven, until lightly browned.