Brown the onions in the fat, sprinkle with flour, and stir while it simmers for 10 minutes. Add the soup, preferably heated, and let boil slowly until the onion is very tender. Spread the cheese thickly on the toast, and melt under a quick broiler. (This is better than putting the toast and cheese on the soup and then melting, since the toast stays crisper.) Pour the soup into a hot tureen, cover with the toast, and serve at once.