Baked French Onion Meatballs with Caramelized Onions

Ingredients

• 1/2 cup (45 g) plain breadcrumbs
• 1/3 cup (80 ml) whole milk
• 1 1/2 lb (680 g) ground beef
• 1 large egg
• 1/3 cup (30 g) grated Parmesan cheese
• 1 tablespoon Worcestershire sauce
• 2 cloves garlic, minced
• 1 tablespoon chopped fresh parsley
• 1 teaspoon fresh thyme leaves, or 1/2 teaspoon dried thyme
• 1 teaspoon kosher salt
• 1/2 teaspoon black pepper
• 2 tablespoons unsalted butter
• 1 tablespoon olive oil
• 3 large yellow onions, thinly sliced
• 1/2 teaspoon kosher salt
• 1 teaspoon brown sugar, optional
• 2 tablespoons all-purpose flour
• 2 cups (480 ml) low-sodium beef broth
• 1 teaspoon Dijon mustard
• 1 teaspoon Worcestershire sauce
• 1/2 teaspoon fresh thyme leaves
• 1 1/4 cups (140 g) shredded Gruyere cheese
• Chopped parsley or thyme, for garnish