In a large soup kettle (at least six-quart size), heat 1 tablespoon of the butter and all of the oil. Add beef shanks and brown well on all sides. Add chopped onion, carrots, salt, pepper, parsley, bay leaf, and water. Bring to a boil, reduce heat, cover, and simmer for about 3 hours, until broth is richly flavored.

Strain broth into a large bowl. When meat is cool enough to handle, cut into chunks and return it, with marrow from bones, to broth; discard vegetables and bones. Chill soup; skim off fat. (This much can be done 1 to 2 days ahead.)

In same large soup kettle, heat remaining 5 tablespoons butter. Add sliced onions, cover, and cook until limp, about 10 minutes. Uncover and cook over low heat, stirring frequently, until onions brown lightly, about 30 minutes longer. (Reduce heat if onions become too brown.) Stir in garlic and flour. Cook and stir until bubbly. Remove from heat and stir in about 4 cups of the broth. Return to heat; mix in remaining broth with beef; add wine. Bring to a gentle boil, cover, reduce heat, and simmer until ready to serve. Salt to taste.

Meanwhile, place bread in a single layer on a baking sheet in a 250 degree oven. Toast until bread is very dry and barely browned (about 30 minutes). Sprinkle about 1/3 cup of the mixed cheeses over each toast slice.

To serve, ladle soup into heatproof bowls. Top each with a cheese-and-toast slice. Place under broiler, about 5 inches from heat, until cheese is melted and lightly browned, 3 to 5 minutes. Serve immediately.

a rich beef broth, dense with onions, crowned with cheese-laden toast, and so filling it is almost a main dish. Here, to take it one rich step further, the broth is made with meaty beef shanks. All this soup needs to become a meal is a leafy salad with an oil and vinegar dressing, and a robust red wine. The broth can be prepared a day or more in advance.