French Onion Meatballs

Name: French Onion Meatballs
Prep time: 15 min
Cook time: 1 h 10 min
Total time: 1 h 25 min
Cuisine: American
Ingredients: 2 tablespoons olive oil; 4 medium yellow onions, thinly sliced (about 6 cups); 3 cloves garlic, thinly sliced; 2 bay leaves; 1 1/2 teaspoons kosher salt; 1 teaspoon fresh thyme leaves; 1/2 teaspoon freshly ground black pepper; 1/3 cup dry white wine, such as sauvignon blanc; 2 tablespoons unsalted butter; 1 pound 90% lean ground beef; 1 large egg, beaten; 1/3 cup panko breadcrumbs; 1 teaspoon kosher salt; 1 tablespoon all-purpose flour; 2 cups low-sodium chicken broth; 1 teaspoon Dijon mustard; 1 teaspoon Worcestershire sauce; 6 ounces freshly grated Gruyere cheese (about 1 cup); Toasted baguette slices, for serving