Do not overmix the meatball mixture or they will become dense and tough. Mix just until combined. Searing the meatballs before baking is highly recommended for the best flavor and caramelized exterior. Warm your marinara before adding the meatballs to ensure even baking. Adjust the gochujang and red pepper flakes to control the heat level. These meatballs taste even better the next day as the flavors deepen overnight.