Best French Onion Meatball Sub Recipe - How to Make Meatball Sub

Ingredients

• 1 lb. ground beef
• 1/2 c. panko bread crumbs
• 1/4 c. finely grated Parmesan (about 2 oz.)
• 1 large egg, beaten
• 3 cloves garlic, finely chopped
• 2 tbsp. chopped fresh parsley
• 2 tsp. fresh thyme leaves
• Kosher salt
• Freshly ground black pepper
• 3 tbsp. unsalted butter
• 3 large yellow onions, thinly sliced
• 2 cloves garlic, minced
• 1/4 c. red wine
• 1 c. low-sodium beef broth
• 4 hoagies, split open
• 1 1/2 c. shredded Gruyère (about 3 oz.)