Remove the onion ends, slice in half length wise, and cut into ¼” thick slices.
Add olive oil to a large pot and heat over high heat.
Add onion, thyme and bay leaves and sauté on high heat until onion is translucent and soften, about 5 minutes.
Season with a few pinches of salt.
Reduce heat to medium high and continue to cook the onion until soft and caramelized, stirring frequently.
As the onion gets closer to being caramelized, the bottom of the pan may start to burn, add a little bit of the beef broth to deglaze it as the onion continues to cook.
When the onion is a deep golden caramel color, about 25 minutes, add the rest of the beef broth to the pot.
Bring to a boil and simmer for another 10 minutes, season with salt and pepper to taste.
Shred or grate the cheese, slice to bread into ¼” - ½” round slices that will fit the mouth of a ramekin or oven safe soup crock.
Remove thyme and bay leaves from the soup, and divide into your container of choice. I used ramekins (see photo above)
When ready to eat, place a slice of bread on top of your soup and top generously with grated Fontina cheese.
Broil for 4 minutes until cheese is melted and golden. (See photo above)
Remove from oven and serve immediately.