In a roasting pan, roast the soup bones in a preheated 450°F oven for about 35 minutes. While roasting the bones, bring the water to a boil and maintain the boil, covered, until the bones are ready. Add the roasted bones to the boiling water, discarding the rendered fat that will have collected on the roasting pan. Simmer the bones, covered, for a minimum of 2 hours.

While the bones are simmering, melt the butter and both oils in a large frying pan or brazier. Brush the tops of the bread slices with the butter mixture, spread the slices on a baking sheet, and put them in a preheated 300°F oven to toast, for about 20 minutes. Set aside to cool. (These will become the croutons to be used later in the soup.)

In a large frying pan or heavy-bottomed pot, sauté the onions in the remaining butter-oil mixture over medium-high heat until they become soft and translucent. Add the sugar, and continue to cook until the onions caramelize (they will turn golden brown). This should take about 30 minutes. Do not leave the onions during this stage, but stir them steadily. When the onions turn a deep golden brown, stir in the flour until it disappears. Remove the pan from the heat and set it aside.

Remove the bones from the broth with a slotted spoon. Using a gravy separator or soup skimmer, strain out all of the fat from the broth. Transfer the onion mixture to the broth. Add the wine, pepper, and salt. Adjust the seasonings to taste.

Preheat your oven broiler to high. Place 1 crouton on the bottom of each soup bowl. (This soup should be served in individual crock-style bowls, with a narrower width, rather than in a large serving bowl or wide-mouthed soup bowls.) Ladle the soup over the croutons, filling the bowls to ½ inch from the top. Float another crouton on top. Cover the bowls with sliced cheese. Place the bowls under the broiler (or into a hot oven if you don’t have a broiler) long enough for the cheese to melt over the top of the soup. Serve immediately.