For the meatballs:

• 1 pound lean ground beef (85/15 or 90/10)
• ½ cup Italian seasoned breadcrumbs
• ¼ cup milk
• 1 large egg
• ¼ cup grated Parmesan cheese
• 2 cloves garlic, minced
• 1 teaspoon fresh thyme leaves
• 1 teaspoon salt
• ½ teaspoon black pepper
• 2 tablespoons olive oil, for browning

For the French onion sauce:

• 2 large yellow onions, thinly sliced
• 2 tablespoons butter
• 1 tablespoon olive oil
• ½ teaspoon salt
• 2 cloves garlic, minced
• 1 tablespoon all-purpose flour
• ¼ cup dry sherry or white wine
• 2 cups low-sodium beef broth
• 1 teaspoon fresh thyme leaves
• 1 cup shredded Gruyère cheese
• ½ cup shredded mozzarella cheese

1. Prepare the Meatballs:
2. In a large bowl, mix together the ground beef, breadcrumbs, milk, egg, Parmesan cheese, minced garlic, thyme, salt, and pepper. Once fully combined, shape the mixture into 1 ½-inch meatballs.
3. Brown the Meatballs:
4. Heat 2 tablespoons of olive oil in a large oven-safe skillet over medium-high heat. Add the meatballs and cook for about 6-8 minutes, turning occasionally to brown them on all sides. Once browned, transfer the meatballs to a plate and set aside.
5. Caramelize the Onions:
6. Reduce the heat to medium-low and add butter and olive oil to the same skillet. Toss in the sliced onions and salt, stirring frequently. Cook until the onions are soft and caramelized, about 25-30 minutes. This slow-cooking process gives the onions their deep, sweet flavor.
7. Create the Sauce:
8. Add the minced garlic to the onions and cook for an additional minute. Sprinkle in the flour and stir constantly for about a minute, allowing the raw flour taste to cook out. Pour in the sherry or white wine, simmering until it’s nearly evaporated, about 1 minute. Then, stir in the beef broth and thyme leaves, bringing the sauce to a gentle simmer.
9. Combine Meatballs and Sauce:
10. Nestle the browned meatballs into the sauce, ensuring they’re evenly distributed. Sprinkle Gruyère and mozzarella cheese generously over the meatballs.
11. Broil to Perfection:
12. Preheat your oven's broiler to high. Place the skillet under the broiler for 2-3 minutes or until the cheese is melted and bubbly.
13. Serve Hot:
14. Garnish with fresh thyme and serve these French Onion Meatballs piping hot. Pair with crusty bread to soak up all that delicious sauce, or serve over mashed potatoes for a full meal.