Thinly slice the yellow onion into half-moons and mince the garlic.
In a medium pot over medium heat, melt the ghee and add the onions with a pinch of sea salt.
Cook the onions for 15-20 minutes, stirring occasionally, until they are deep golden brown and caramelized.
Add the minced garlic and the thinly sliced beef sirloin to the pot, searing the beef until browned.
Pour in the beef bone broth and balsamic vinegar, then add the fresh thyme sprig and black pepper.
Bring the soup to a gentle simmer and let the flavors meld for 10 minutes.
Remove the thyme sprig and ladle the soup into an oven-safe crock or bowl.
Top the soup with the shredded Gruyere cheese.
Place the bowl under a broiler for 2-3 minutes until the cheese is melted, bubbly, and slightly browned.
Carefully remove from the oven and serve immediately while hot.