In a large mixing bowl, combine ground beef, breadcrumbs, egg, salt, and pepper. Mix until combined and shape into 1-1.5 inch meatballs. Heat a skillet with olive oil and brown the meatballs for 5-7 minutes.

In the same skillet, add sliced onions and sauté over medium-low heat for 20-25 minutes until golden brown. Optional: add sugar midway for enhanced caramelization.

Pour in the beef broth into the caramelized onions and bring to a simmer. Return the browned meatballs to the skillet, allowing them to simmer for 10 minutes.

Sprinkle Gruyère cheese on top of the meatballs, cover with a lid, and let cook on medium-low for 5 minutes until cheese is melted and bubbly.