In a large skillet, melt the butter over medium heat. Add the sliced sweet yellow onions and cook until translucent, about 10 minutes.

Reduce the heat to low and continue to cook the onions for approximately 30 minutes until golden brown.

Pour in the beef broth and add the thyme. Simmer for 10 minutes, scraping bits from the skillet.

Preheat your oven to 350°F (175°C).

In a baking dish, layer half of the baguette slices followed by half of the onion mixture and half of the cheese.

Repeat the layering process with the remaining baguette, onion mixture, and top with the rest of the cheese.

Bake uncovered for 25-30 minutes until bubbly and golden brown.

Let the casserole cool slightly before serving, optionally garnishing with fresh thyme.