In a large mixing bowl, add all ingredients and mix together gently with a spatula or your hands. Mix just until incorporated, as overmixing will cause the meatballs to be tougher.
1 pound lean ground beef, ½ pound ground pork, 1 egg, ½ cup Italian seasoned breadcrumbs, ⅓ cup parmesan cheese, 2 teaspoons kosher salt, 1 teaspoon black pepper, ½ teaspoon red pepper flakes, 3 tablespoons green onions