Heat the extra virgin olive oil in a medium pot over medium-low heat.
Add the thinly sliced yellow onion and cook, stirring occasionally, for 20-25 minutes until deep golden brown and caramelized.
Increase the heat to medium and add the cubed beef sirloin, searing until browned on all sides.
Pour in the beef bone broth and add the dried thyme, bay leaf, sea salt, and black pepper.
Bring the soup to a gentle simmer and cook for 15 minutes to allow the flavors to develop.
While the soup simmers, lightly toast the sourdough bread slice.
Remove the bay leaf from the soup and ladle the mixture into an oven-safe bowl or crock.
Place the toasted bread on top of the soup and sprinkle evenly with the shredded Gruyere cheese.
Place the bowl under a broiler for 2-3 minutes until the cheese is melted, bubbly, and slightly browned.