In a heavy 4- to 5-quart saucepan or a soup kettle, melt the butter with the oil over moderate heat. Stir in the onions and 1 teaspoon salt, and cook uncovered over low heat, stirring occasionally, for 20 to 30 minutes, or until the onions are a rich golden brown. Sprinkle flour over the onions and cook, stirring, for 2 to 3 minutes. Remove the pan from the heat. In a separate saucepan, bring the beef stock and wine to a simmer, then stir the hot stock into the onions. Return the soup to low heat and simmer, partially covered, for another 30 to 40 minutes, occasionally skimming off the fat. Taste for seasoning, and add salt and pepper if needed.