1. Preheat oven to 375°F (190°C). Mix ground beef, breadcrumbs, egg, garlic powder, thyme, salt, and pepper. Form into meatballs and bake for 20 minutes.
2. In a skillet, melt butter and cook onions on low heat until caramelized, about 20–25 minutes. Sprinkle flour and stir for 1–2 minutes.
3. Add beef broth and simmer until the gravy thickens. Place cooked meatballs into the skillet.
4. Top meatballs with shredded Gruyère cheese and broil in the oven for 3–5 minutes until cheese is bubbly and golden.
5. Garnish with thyme and serve hot with crusty bread.