Prepare the Panade: In a small bowl, combine the breadcrumbs and milk. Let it sit for 5 to 10 minutes until the bread soaks up the liquid and forms a thick paste.
Saute the Aromatics: Heat a teaspoon of olive oil in a skillet over medium heat. Add the chopped onion and cook for 3 to 4 minutes until soft. Add garlic and cook 30 seconds more. Let cool slightly.
Mix the Meatball Mixture: In a large mixing bowl, combine the ground beef, ground pork, cooled onion-garlic mixture, soaked breadcrumbs, egg, parsley, salt, pepper, oregano, and red pepper flakes. Use your hands to gently mix just until combined. Do not overwork.
Shape the Meatballs: Scoop out about 2 tablespoons of mixture per meatball. Roll gently between your palms to form smooth balls. Wet your hands slightly with water to prevent sticking.
Sear the Meatballs: Heat the remaining olive oil in a large skillet over medium-high heat. Add the meatballs in a single layer and sear, turning, until browned on all sides.
Finish Cooking: Simmer seared meatballs in marinara sauce for 15 to 20 minutes, or bake at 375F (190C) for 15 to 18 minutes, until cooked through.