French Onion Soup in Casserole

Ingredients:

• 3 tablespoons unsalted butter
• 2 tbsp. l. olive oil
• 4 medium onions, thinly sliced
• 2 stalks celery, peeled and thinly sliced
• 2 bay leaves
• 2-3 sprigs of thyme + 1 teaspoon of fresh leaves
• 1 tbsp flour
• 1 cup lightly salted beef broth
• 0.5 cups dry white wine
• Half a baguette, thinly sliced ​​(about 100g)
• 100 g grated Gruyere (about 2 cups)
• Special equipment: 3-liter baking dish.

Preparation:

1. In a large Dutch oven or saucepan, melt the butter and olive oil over medium heat. Add half the onion and celery, 1/2 teaspoon salt, and a pinch of freshly ground black pepper. Top with the remaining onion and celery, 1/2 teaspoon salt, a pinch of freshly ground black pepper, a bay leaf, and thyme sprigs.
2. Cook, uncovered, stirring occasionally, until all the liquid has evaporated and the onions are very soft and golden brown, 45 minutes to 1 hour. If the onions are soft but not golden enough, increase the heat slightly for a few minutes toward the end of cooking.
3. 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 heat and let cool for a few minutes. Remove the bay leaf and thyme sprigs. The onions can be cooked, cooled, and stored in the refrigerator for up to 2 weeks.
4. Meanwhile, preheat the oven to 350°F (175°C). Bake the baguette slices on a baking sheet until lightly toasted but not golden, 2-3 minutes per side. Let cool.
5. Place half the bread slices, slightly overlapping, in a 3-quart (quart) ovenproof dish. Spread the onion mixture on top in a single layer. Layer the bread slices again, slightly overlapping. Sprinkle with Gruyère. Cover with foil and bake until the onion mixture is heated through and the cheese is melted, about 40 minutes. Remove the foil, turn the oven to broil, and broil until golden brown. Sprinkle with thyme leaves. Serve the casserole hot.