In a large bowl, combine ground beef, Italian sausage, Parmesan cheese, breadcrumbs, egg, parsley, garlic, milk, oregano, salt, black pepper, and red pepper flakes. Mix until just combined.

Shape the mixture into 1-inch balls, aiming for about 20-24 meatballs.

Heat olive oil in a skillet over medium-high heat, sear the meatballs for 2-3 minutes on each side until golden brown.

In the same skillet, sauté chopped onion until softened, then add minced garlic and cook until fragrant.

Add crushed tomatoes, basil, oregano, and sugar to the skillet. Bring to a simmer, and then add the meatballs. Cover and simmer for 20-25 minutes.

Remove meatballs from sauce and cool slightly. Soak wooden skewers in water for 10 minutes if grilling.

Thread meatballs and mozzarella onto soaked skewers, alternating each.

Garnish assembled skewers with basil leaves and serve warm, with sauce on the side.