Preheat the oven to 350°F (175°C).
In a large skillet, melt the butter and olive oil over medium heat.
Add the sliced onions to the skillet and season with salt and pepper. Sauté for 20-25 minutes, stirring often, until the onions are caramelized.
Stir in the thyme, followed by the heavy cream and beef broth. Mix well and cook for an additional 5 minutes.
Transfer the mixture to a baking dish and spread it out evenly. Top with cubed French bread and shredded Gruyère cheese.
Bake in the preheated oven for 30-35 minutes, or until the top is melted and golden brown.
Remove from the oven and allow it to cool for a few minutes before serving.