Preheat & prep: Take the meat out of the fridge about 15 minutes before you start so it's not ice-cold.
Make the panade: In a small bowl, tear the bread into small pieces and pour the milk over it. Let it sit for 5 minutes, then mash it with a fork into a paste. It should look like wet oatmeal.
Combine the meat mixture: In a large bowl, combine the beef, pork, panade, Parmesan, egg, garlic, parsley, salt, pepper, and red pepper flakes if using. Use your hands and mix gently until just combined, about 30 seconds. Don't squeeze or knead.
Form the meatballs: Lightly wet your hands to keep the mixture from sticking. Roll into golf-ball-sized balls — about 1.5 tablespoons each. Place them on a sheet pan. You should get about 16–18 meatballs.
Sear the meatballs: Heat the olive oil in the skillet over medium-high heat until shimmering. Add the meatballs in a single layer (work in batches if needed). Cook for 3–4 minutes per side, turning gently, until deeply browned on at least two sides. Don't move them too early — let the crust form.
Simmer in sauce: Reduce the heat to low. Pour the marinara sauce around the meatballs (not on top). Cover and simmer for 15–20 minutes, until the meatballs are cooked through and the sauce has thickened slightly.
Finish and serve: Spoon the sauce and meatballs onto a platter or into a shallow bowl. Top with a few fresh basil leaves and a shower of Parmesan. Let them rest for about 5 minutes before serving.