4 1 ½-inch thick slices baguette
6 tablespoons (¾ stick) unsalted butter
4 large yellow onions, julienned
¼ cup red wine
2 tablespoons ruby port
1 ½ quarts beef broth
1 bay leaf
1 sprig thyme
1 teaspoon balsamic vinegar
Freshly ground black pepper
½ pound shredded Gruyère
Preheat the oven to 375˚F. Arrange the baguette slices in a single layer on a baking tray and bake for 15 to 20 minutes, or until dry. Let cool.
Heat a large, heavy pot over medium-low heat. Add the butter and onions and cook, stirring frequently, for 1 hour to 1 hour and 15 minutes, or until caramelized. Add the wine and port and simmer for a minute or so, scraping up the brown bits from the bottom of the pot with a heat-proof spatula. Add the broth, bay leaf, and thyme. Bring to a boil and simmer for 40 to 45 minutes, or until the flavors come together. Discard the bay leaf and thyme sprig. Stir in the balsamic vinegar and season to taste with salt and pepper. Preheat the broiler. Divide the soup among 4 oven-proof soup bowls and arrange the soup bowls on a baking tray. Press 1 baguette slice cut side up into the top of each one and divide the Gruyère among them. Broil for 3 to 4 minutes, or until the Gruyère is melted and golden brown. Serve immediately.
Makes 4 very generous servings.