Begin by melting 4 tablespoons of unsalted butter in a large skillet over medium heat. Add the 4 sliced large onions along with a pinch of salt. Cook for 20-25 minutes, stirring occasionally, until caramelized and golden brown.

Once your onions are caramelized, pour in 2 cups of beef broth (or vegetable broth), along with 1 teaspoon of thyme and freshly cracked pepper to taste. Stir and let simmer for about 5 minutes.

Reduce heat to low and stir in 1 cup of heavy cream until well integrated. Remove from heat and set aside.

Preheat your oven to 350°F (175°C). In a baking dish, layer half of the cubed French bread, followed by half of the creamy onion mixture, and then half of the grated Gruyère cheese. Repeat each layer until all ingredients are used, ending with cheese.

Cover the casserole with aluminum foil and bake for 25 minutes. Remove foil and bake for an additional 15-20 minutes, until the top is golden brown and bubbly.

Let the casserole rest for about 10 minutes before serving.