3 tablespoons butter
1 tablespoon olive oil
2 medium leeks, white and light green parts only, halved lengthwise and thinly sliced
2 medium red onions (about 1 3/4 pounds total), halved and thinly sliced
| 2 medium yellow onions (about 1 3/4 pounds total), halved and thinly sliced
.1 teaspoon coarse kosher salt
1 tablespoon all-purpose flour
3/4 cup dry red wine
1 quart beef broth
1/4 teaspoon freshly ground black pepper
3 sprigs fresh thyme
1 bay leaf
Garlic Croutons (recipe follows)
6 ounces Gruyere cheese, coarsely grated (about 1 1/2 cups)
In a large, heavy pot that is wider than it is deep, heat butter and olive oil over medium-high. Add leeks, onions and salt and stir well. Cook, stirring occasionally, until onions soften, release moisture, and are just beginning to brown, about 30 minutes.
Reduce heat to medium and continue to cook, stirring often and scraping up any browned bits that have adhered to bottom of pot, until onions are very soft, brown and caramelized, about 30 minutes more. Stir in flour, cook one minute; add wine and pepper, cook one minute more. Add beef broth, thyme and bay leaf. Bring to a boil, reduce heat to medium low and simmer, covered, for 10 minutes.
Heat broiler with rack positioned 5 inches below heat source. Ladle soup into six ovenproof bowls; top each with one garlic crouton and about 1/4 cup cheese. Place three soup bowls on a baking sheet and broil until cheese is browned and bubbly, about 2 to 3 minutes. Repeat with remaining three bowls of soup.
To make garlic croutons: Heat broiler. Brush 6 slices of baguette with olive oil on both sides and season with salt and pepper. Place on baking sheet and broil until golden, about 1 minute; remove from oven and turn bread slices over. Return to oven and broil 1 to 2 minutes more. Remove from oven and rub toasts with the cut side of half a clove of garlic.