Step 1

In a large bowl, combine the ground beef, breadcrumbs, milk, egg, Parmesan, garlic, thyme, salt and pepper. Mix until well combined. Shape into 1 1/2-inch meatballs.

Step 2

Heat the olive oil in a large oven-safe skillet over medium-high heat. Add the meatballs and cook, turning occasionally, until browned on all sides, about 6-8 minutes. Transfer to a plate.

Step 3

Reduce the heat to medium-low. Add the butter and olive oil to the skillet. Stir in the sliced onions and salt. Cook, stirring often, until the onions are very soft and caramelized, about 25-30 minutes.

Step 4

Add the garlic and cook 1 minute more. Stir in the flour and cook, stirring constantly, for 1 minute.

Step 5

Pour in the sherry and simmer until nearly evaporated, about 1 minute. Stir in the beef broth and thyme leaves. Bring to a simmer.

Step 6

Nestle the meatballs into the onion sauce. Sprinkle the Gruyère and mozzarella cheeses on top.

Step 7

Broil on high until the cheese is melted and bubbly, about 2-3 minutes. Garnish with fresh thyme and serve hot with crusty bread or over mashed potatoes.