In a large mixing bowl, combine ground beef, bread crumbs, and egg, seasoning with salt and pepper. Use your hands to mix until just combined. Form the mixture into meatballs, about 1 to 1.5 inches in diameter, and set aside on a plate.

Heat a large skillet over medium-low heat and add olive oil. Once hot, add the sliced onions, stirring occasionally. Cook for 25 to 30 minutes until golden brown.

In the same skillet, add the prepared meatballs. Increase the heat to medium and brown the meatballs on all sides, about 5 to 7 minutes.

Once browned, return the caramelized onions to the skillet. Pour in the beef broth and scrape up any browned bits. Bring to a gentle simmer for 5 minutes.

Sprinkle Gruyère cheese over the top of the meatballs and onions. Cover and cook on low heat for about 5 minutes, until cheese is melted.

Transfer the Cheesy French Onion Meatballs onto a serving platter and spoon some of the onion sauce over them. Serve hot.