Thinly slice the yellow onion and mince the garlic clove.
In a medium pot over medium heat, melt the ghee and add the sliced onions, cooking for about 15-20 minutes until they are deeply caramelized and golden brown.
Cut the lean beef sirloin into small half-inch cubes and add them to the pot with the onions, searing until browned on all sides.
Stir in the minced garlic, dried thyme, sea salt, and black pepper, cooking for one minute until the spices become fragrant.
Pour in the beef bone broth and bring the mixture to a gentle simmer; cover and cook for 15 minutes to allow the flavors to meld and the beef to become tender.
While the soup simmers, toast the sourdough bread slice in a toaster or under a broiler until it is very crisp.
Ladle the hot soup into an oven-safe crock or bowl, place the toasted sourdough on top, and sprinkle with the shredded Gruyere cheese.
Place the crock under the oven broiler for 2-3 minutes until the cheese is melted, bubbly, and shows small golden-brown spots.