In a large bowl, combine the ground beef, almond flour, egg, Worcestershire sauce, garlic powder, salt, and pepper. Mix gently until just combined, then shape into 16 meatballs. Set aside.
Melt the butter in a large oven-safe skillet over medium heat. Add the sliced onions and cook, stirring occasionally, for 15 to 18 minutes until they are deeply caramelized and golden brown. Sprinkle in the thyme leaves and stir.
Push the onions to the edges of the skillet and increase the heat to medium-high. Sear the meatballs in the center for about 2 minutes per side until nicely browned on the outside. They do not need to be cooked through at this point.
Pour the beef broth and Worcestershire sauce into the skillet around the meatballs. Stir gently to combine with the onions. Bring to a simmer, then reduce heat to medium-low. Cover and let the meatballs simmer for 12 to 15 minutes until cooked through.
Preheat your broiler to high. Sprinkle the shredded gruyere cheese generously over the top of the meatballs and onions in the skillet.
Place the skillet under the broiler for 2 to 3 minutes, watching closely, until the cheese is melted, bubbly, and lightly golden. Garnish with fresh thyme and serve straight from the skillet.