Ingredient

For the Onions

• 3 large onions, thinly sliced, about 6 cups
• 2 large cloves garlic, minced
• 1 large bay leaf
• 1 tablespoon unsalted butter
• 1 tablespoon balsamic vinegar
• 2 teaspoons Worcestershire sauce
• 1 teaspoon fine salt
• Freshly ground black pepper, to taste

For the Meatballs

• 1 1/2 pounds lean ground beef (85/15 or 90/10)
• 1/2 cup panko breadcrumbs
• 1 packet (1 ounce) dry onion soup mix
• 1 large egg
• 1/2 teaspoon herbes de Provence or dried thyme

For the Sauce and Finishing

• 1 (10.5-ounce) can condensed French onion soup
• 1 cup unsalted beef broth or water
• 1/2 teaspoon herbes de Provence or dried thyme
• 1 1/2 to 2 cups (6 to 8 ounces) shredded Gruyère or Swiss cheese
• Fresh thyme leaves or fresh chopped parsley, for garnish, optional
• 1 baguette, sliced and toasted

Direction

1. Gather the ingredients.
2. Combine 3 large onions, thinly sliced (about 6 cups), 2 large cloves garlic, minced, 1 large bay leaf, 1 tablespoon unsalted butter, 1 tablespoon balsamic vinegar, 2 teaspoons Worcestershire sauce, 1 teaspoon fine salt, and freshly ground black pepper to taste in a large slow cooker and give everything a stir to combine. Cover and cook on high for 3 hours, stirring again after 1 hour.
3. About 25 minutes before the 3 hours is up, prepare the meatballs. Position a rack in the center of the oven and heat to 425 F. Line a large rimmed baking sheet with foil. In a large bowl, combine 1 1/2 pounds lean ground beef (85/15 or 90/10) with 1/2 cup panko breadcrumbs, 1 packet (1 ounce) dry onion soup mix, 1 large egg, and 1/2 teaspoon herbes de Provence or dried thyme. 
4. Mix until the ingredients are combined (but do not overmix) and using 2 tablespoons or a 2-tablespoon portion scoop, shape the mixture into 24 to 28 1-ounce meatballs and place on the prepared baking sheet.
5. Bake the meatballs in the oven for 10 minutes.
6. Add the meatballs, 1 can (10.5 ounces) condensed French onion soup, 1 cup unsalted beef broth or water, and 1/2 teaspoon herbes de Provence or dried thyme to the slow cooker. 
7. Cover and cook on high for 2 hours. Taste and adjust the seasonings. Turn the slow cooker to warm until it’s time to serve the meatballs.
8. Just before serving, sprinkle the meatballs evenly with 1 1/2 to 2 cups (6 to 8 ounces) shredded Gruyère cheese. Cover and let the meatballs stand until the cheese is melted, 5 to 10 minutes.
9. Garnish the meatballs with fresh thyme leaves or chopped parsley if desired, and serve them with 1 baguette, sliced and toasted.