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

Once the onions are beautifully caramelized, stir in the beef broth, fresh thyme, and a dash of pepper. Increase the heat slightly and allow the mixture to simmer for about 5 minutes.

Lower the heat and gently mix in the heavy cream, stirring well to combine. Continue to cook for an additional 2-3 minutes, then remove it from the heat.

Preheat your oven to 350°F (175°C). In a greased baking dish, layer half of the cubed French bread at the bottom. Pour half of the onion mixture over the bread. Sprinkle half of the Gruyère cheese on top.

Cover the assembled casserole with aluminum foil and bake for 25 minutes. After this time, carefully remove the foil and continue baking for an additional 15 to 20 minutes, or until the top is golden and bubbling.

Once baked, remove from the oven and let it rest for about 10 minutes before serving.