Preheat the oven to 400°F (200°C). Spray a 9×13 inch baking dish with cooking spray.
Melt butter in a large pan over medium heat. Add onions and cook until soft, sweet, and golden brown (about 15 minutes). Add garlic, salt, pepper, and thyme; cook for 1 more minute.
Sprinkle flour over the onions, cook for a couple of minutes, then pour in the wine. Let it simmer until the liquid reduces slightly. Add both broths and bring to a simmer for 10 minutes.
Pour half the onion mixture into the baking dish. Layer half the toasted bread slices, then sprinkle with half of the Gruyère and Parmesan. Repeat with the remaining ingredients.
Bake uncovered for 20-25 minutes, or until the cheese is bubbly and browned at the edges.
Let it rest for a few minutes, then garnish with fresh parsley before serving.
Variations & Tips:
For a vegetarian version, replace the beef broth with chicken or vegetable broth.
Experiment with different cheeses for new flavors.
Use gluten-free bread for a gluten-free casserole.
Prep the casserole a day ahead and refrigerate until ready to bake.
Short on time? Use store-bought caramelized onions.
This French Onion Soup Casserole is not just a meal; it’s an experience. A perfect blend of tradition and modernity, it’s sure to be a hit at your dinner table. Enjoy the cooking, and savor the delicious moments!