French Onion Soup Casserole
2 Tablespoons olive oil
4 medium onions, thinly sliced
2 medium stalks celery, peeled and thinly sliced
1 teaspoon kosher salt
½ teaspoon freshly ground black pepper
2 bay leaves
2 sprigs fresh thyme, plus 1 teaspoon fresh leaves
1 Tablespoon all-purpose flour
1 cup low-sodium beef broth
½ cup dry white wine
½ baguette, thinly sliced (about 4 ounces)
4 ounces Gruyere cheese, shredded
Melt the butter and oil in a large Dutch oven or pot over medium heat. Add half the onions and celery, ½ teaspoon salt and ¼ teaspoon pepper. Top with the remaining onions and celery, ½ teaspoon salt, ¼ teaspoon pepper, the bay leaves and the thyme sprigs. Cook, uncovered and stirring occasionally, until all the liquid evaporates and the onions are very soft and golden brown, 45 minutes to 1 hour. (If the onions are soft but could be more golden, increase the heat slightly for a few minutes toward the end of cooking.)
Add the flour, and stir until completely absorbed, about 1 minute. Add the broth and wine, and simmer until almost completely evaporated, about 2 minutes. Remove from the heat, and let cool for a few minutes. Remove the bay leaves and thyme stems. (The onions can be made, cooled and refrigerated up to 2 days ahead.)
Meanwhile, preheat the oven to 350° F. Bake the baguette slices on a baking sheet until they're slightly toasted but not golden, 2 to 3 minutes per side. Let cool.
Shingle and slightly overlap half the bread slices in a 3-quart flameproof baking dish. Top with the onion mixture in 1 layer. Shingle, overlapping, with the remaining bread. Sprinkle the Gruyere over the top. Cover with foil, and bake until the onion mixture is hot and the cheese is melted, about 40 minutes. Remove the foil, switch the oven from bake to broil and broil until the top is golden in spots. Sprinkle the thyme leaves over the top. Serve hot.
Makes 8 servings.