In a heavy kettle cook the onions in the oil over moderately high heat, stirring occasionally, for 1 hour, or until they are golden brown. Stir in the cider, the broth, the water, the brandy, and salt and pepper to taste and simmer the soup, uncovered, for 20 minutes. Arrange the toast slices on a baking sheet, sprinkle the Roquefort on them, and broil the croutons under a preheated broiler about 4 inches from the heat until the cheese begins to melt. Ladle the soup into heated bowls and float a Roquefort crouton in each serving. Makes about 7 cups, serving 4 to 6 as a main course.