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FOR THE SOUP:
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Melt the butter and oil in a large heavy stockpot over medium heat. Add the onions and 1 tablespoon salt, reduce the heat to low. Cook, stirring every 15 minutes and regulating the heat to keep the mixture bubbling gently, for about 1 hour, or until the onions have wilted and released a lot of liquid. At this point, you can turn up the heat slightly to reduce the liquid, but it is important to continue to cook the onions slowly to develop maximum flavor and keep them from scorching. Continue to stir the onions every 15 minutes, being sure to scrape the bottom and get in the corners of the pot, for about 4 hours more, or until the onions are caramelized throughout and a rich deep brown. Keep a closer eye on the onions toward the end of the cooking when the liquid has evaporated. Remove from the heat. (You will need 1 1/2 cups of onions for the soup; reserve any extra for another use. The onions can be made up to 2 days ahead.
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Transfer the caramelized onions to a 5 quart Sift in the flour and cook over medium-high heat, stirring, for 2 to 3 minutes. Add the beef stock and sachet, bring to a simmer, and simmer for about 1 hour, or until the liquid is reduced to 2 1/2 quarts. Season to taste with salt, pepper, and a few drops of vinegar. Remove from the heat.
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CROUTONS: Cut 3/8 inch thick slices from the baguette and place on a baking sheet. Brush with olive oil and sprinkle lightly with salt. Broil
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TO COMPLETE: Return the soup to a simmer. On a baking sheet to catch any spills the soup will bubble up. Add the hot soup to the tureens, filling them within 1/2 inch of the tops. Top each serving with 2 croutons: Lay them on the surface - do not push them into the soup. Lay the slices of cheese over the croutons so that the cheese overlaps the edges of the tureens by about 1/2 inch, Scatter the grated cheese over the sliced cheese, filling in any areas where the sliced cheese is thiner, or it may melt into the soup rather than forming a crust.
