In a large skillet, melt the butter and olive oil over medium heat. Add the sliced onions and cook, stirring occasionally, for about 20-25 minutes or until they turn golden brown and caramelized.

Once the onions are nicely caramelized, return the heat to medium-high, and add the beef or vegetable broth, dried thyme, salt, and pepper. Let this simmer for about 5 minutes.

Reduce the heat to low, and slowly stir in the heavy cream until mixed well and warm.

Preheat your oven to 350°F (175°C). In a large baking dish, layer half of the onion mixture, then sprinkle one cup of shredded cheese. Layer the remaining onion mixture on top, and top it all with the remaining cup of cheese and your croutons or toasted bread.

Place in the preheated oven and bake for about 25-30 minutes or until bubbly and golden on top.

Once out of the oven, let it cool for a few minutes before digging in.