In a large mixing bowl, combine ground beef, breadcrumbs, and a beaten egg along with your preferred seasonings. Use your hands to blend the ingredients until well incorporated but avoid overmixing. Shape the mixture into meatballs approximately 1 to 1.5 inches in diameter.

Heat a tablespoon of oil over medium heat in a large skillet. Place the meatballs into the skillet in a single layer, searing for about 5-6 minutes until browned on all sides.

In the same skillet, add sliced onions and cook slowly on medium-low heat for about 15-20 minutes, stirring occasionally until caramelized. Sprinkle sugar halfway through cooking.

Add beef broth to the skillet, scraping up brown bits. Bring to a simmer and cook for 2-3 minutes. Return meatballs to skillet, cover, and let simmer for an additional 10-12 minutes.

Take skillet off the heat and sprinkle Gruyère cheese over meatballs. Cover to allow cheese to melt for about 3-5 minutes.

Serve meatballs draped in the caramelized onion sauce with sides like creamy mashed potatoes or crusty bread.