In a saucepan, brown the onions in the fat, sprinkle with flour and stir while it simmers for 10 minutes. Deglaze with white wine. Meanwhile heat the consomme. Add it to the onions and let boil slowly until the onions are tender. Spread the cheese thickly on the toast and melt under a quick broiler. Pour the soup into a hot soup tureen, cover with the toast and serve immediately.