In a large mixing bowl, grate one yellow onion and whisk in one egg until blended. Then, add fresh minced garlic, Italian breadcrumbs, freshly grated Parmesan cheese, kosher salt, and black pepper. Mix everything together gently until fully combined.

Carefully fold in 1.5 pounds of ground beef with the meat mixture, being cautious not to overwork it. Once just combined, form the mixture into 24 meatballs, each weighing about 1.5 ounces.

Pour 1.5 cups of marinara sauce into the bottom of your slow cooker, then add the prepared meatballs in a single layer on top of the sauce. Pour the remaining marinara sauce over the meatballs without stirring.

Cover the slow cooker and set it to cook on HIGH for 3-4 hours or LOW for 6-8 hours.

Once cooked, carefully lift the meatballs from the slow cooker, allowing excess sauce to drip off. Serve hot, topped with freshly grated Parmesan cheese and basil.