Preheat your oven to 400°F (200°C).
In a skillet, sauté the chopped onion in a bit of oil until golden brown.
In a large bowl, combine ground beef, breadcrumbs, Parmesan, sautéed onions, garlic, egg, thyme, salt, and pepper.
Mix well until all ingredients are combined.
Form the mixture into meatballs, about 1.5 inches in diameter.
Place meatballs on a baking sheet lined with parchment paper.
Bake in the preheated oven for 20 minutes.
In a saucepan, heat beef broth and add the meatballs once cooked.
Sprinkle shredded Gruyère cheese over the meatballs and cover until melted.
Serve warm, garnished with fresh herbs if desired.