PREPARE THE QUICK-AND-EASY BEEF STOCK on page 198. When the stock is
made and strained, return it to the pot. Set the pot over very low heat, then cover it, and keep the stock warm at a very low simmer while you prepare the soup.
IN A 5-QUART HEAVY POT (WITH A LID) over medium-low heat, heat the
butter and oil. When hot, add the onions. Cover and cook, stirring frequently, 15 minutes.
REMOVE THE LID and raise the heat to medium. Stir in 1 teaspoon salt, the
sugar, and the flour. Cook, stirring constantly, scraping the bottom of the pan so that the flour does not burn, until the onions are rich golden (like the color of light brown sugar), 35 to 40 minutes or more. (While you are cooking the onions, the flour will start to darken too and the onions will cook down considerably. That’s okay.)
WHEN THE ONIONS ARE DONE, add the simmering stock and 1/2 cup of the
wine. Season the soup with salt and pepper, and a pinch or two of extra sugar if desired. Simmer, partially covered with the lid set ajar, 40 minutes more. With a large spoon, skim off any foam that forms. Add the remaining 1/4 cup wine and season the soup again with salt and pepper. (Soup can be prepared three days ahead. Cook to this stage, then cool, cover, and refrigerate. Reheat over medium heat.)
WHILE THE SOUP IS SIMMERING, prepare the baguette slices and the cheese
topping. Arrange a rack at center position of the oven and preheat to 350°F.
BRUSH THE BAGUETTE SLICES generously on both sides with olive oil and
arrange on a rimmed baking sheet. Bake until slices are crisp, 4 to 5 minutes per side. Remove and cool. (Baguette slices can be prepared two days ahead; store in an airtight container at room temperature.) Retain oven temperature.
ARRANGE 6 OVENPROOF SOUP BOWLS or ramekins on a rimmed baking sheet
and fill them 3/4 full with the hot soup. Divide the slivered cheese among the bowls. Float 2 to 3 baguette slices on top of each serving, and sprinkle generously with some grated cheese. Depending on the size of your bowls or ramekins, you may have some soup, cheese, or croutons left over.
BAKE THE SOUPS until the cheese has melted and is lightly browned, 15
minutes. Watch constantly. If desired, run under a hot broiler to brown more, 1 to 2 minutes.