Cooking Instructions

• Arrange one oven rack 6 to 8 inches from the broiler and a second rack in the middle of the oven. Heat the oven to 400°F. Line a rimmed baking sheet with parchment paper or a silicone baking mat.
• Halve and thinly slice the yellow onions. Pick the thyme leaves until you have 2 tablespoons. Mince the garlic cloves and grate the Gruyère cheese.
• In a large bowl, combine the garlic, 2 teaspoons of thyme, ¼ cup of Gruyère cheese, panko breadcrumbs, milk, egg, Worcestershire sauce, 1 ¼ teaspoons of kosher salt, and ¼ teaspoon black pepper. Stir to combine and then add ground beef, mixing gently.
• Form the mixture into 32 to 34 meatballs, placing them on the baking sheet spaced about ¾-inch apart.
• Bake on the middle rack until browned and cooked through, about 15 to 20 minutes.
• Melt butter in a skillet over medium heat. Add onions, 1 tablespoon of thyme, kosher salt, and black pepper. Cook until the onions are deep golden brown, about 25 to 30 minutes.
• Sprinkle flour over onions and stir until incorporated. Add white wine and scrape up browned bits from the skillet. Stir until wine evaporates.
• Add beef broth and whisk to combine. Bring to a boil, cooking until the sauce thickens, about 6 to 8 minutes. Stir in vinegar, adjusting seasoning as needed.
• Remove meatballs from the oven and transfer them to the sauce in the skillet. Coat gently, then spread into an even layer. Sprinkle with remaining Gruyère cheese.
• Broil until cheese is melted and bubbly, about 3 to 4 minutes. Garnish with thyme and serve with crusty bread.