2 ounces (1/4 cup) unsalted butter, more for the baking sheet
4 medium-large yellow onions (about 2 lb.), thinly sliced (8 cups)
1 tablespoon of your favorite brandy
Sea salt and freshly ground black pepper
1 teaspoon granulated sugar
1 small baguette (1/2 lb.), cut into 1/2-inch slices
2 quarts Beef Broth
1 bay leaf
2 cups grated Gruyère cheese
Fresh chopped parsley for garnish (optional)
Melt the butter in a 4-quart pot over medium heat. Stir in the onions and season with 1 tsp. salt and a few grinds of pepper. Remove the onions and add 1 tablespoon of brandy. Cook until brandy has absorbed. Return the onions. Reduce the heat to low. Press a piece of foil onto the onions to cover them completely, cover the pot with a lid, and cook, stirring occasionally (you will have to lift the foil), until the onions are very soft but not falling apart, 40 to 50 minutes. Remove the lid and foil, raise the heat to medium high, and stir in the sugar. Cook, stirring often, until very deeply browned, 10 to 15 minutes.
To make the baguette toasts:
Position a rack in the center of the oven and heat the oven to 350°F. Butter a rimmed baking sheet and arrange the baguette slices on the sheet in a single layer. Bake until the bread is crisp and lightly browned, turning once, 15 to 20 minutes. Set aside.
Add the broth and bay leaf to the caramelized onions and bring the soup to a boil over medium-high heat. Reduce the heat to medium low and simmer for 10 minutes to blend the flavors. Discard the bay leaf and season to taste with salt and pepper.
To serve, position a rack 6 inches from the broiler and heat the broiler to high. Put 6 to 8 broiler proof soup bowls or crocks on a baking sheet. Ladle the hot soup into each each bowl. Top the soup with the baguette toasts. Sprinkle with the cheese and broil until the top is browned and bubbly, 2 to 5 minutes.
Serve with the remaining baguette and herbed butter and a light salad.