Slow Cooker Mozzarella Stuffed Meatballs

Ingredients

• 1 pound lean ground beef
• 1 pound mild Italian sausage
• 1 cup breadcrumbs
• 1 small yellow onion, finely chopped
• 3 cloves garlic, minced
• 1/4 cup Parmesan cheese, grated
• 2 whole eggs
• 1/2 cup whole milk
• 1/2 cup fresh parsley, finely chopped, plus more for serving
• 1/2 teaspoon kosher salt
• 1/2 teaspoon pepper
• 6 sticks mozzarella cheese, cut into 4 pieces each
• 6-8 cups marinara sauce, homemade or store bought
• Kaiser rolls and provolone cheese for serving (optional)