Prep the mix-ins. In a large bowl, combine breadcrumbs, Parmesan, onion, garlic, parsley, Italian seasoning, red pepper flakes, salt, and black pepper. Stir in the egg until the mixture is evenly moistened.
Add the meat. Add the ground turkey and gently mix with a fork or your hands until just combined.
Do not overmix; this keeps the meatballs tender.
Shape the meatballs. Lightly oil your hands and form 16–20 golf ball–sized meatballs. Place them on a plate or sheet pan.
Brown the meatballs (optional but adds flavor). Heat 1–2 tbsp olive oil in a large skillet or Dutch oven over medium heat. Sear the meatballs in batches, turning gently, until lightly browned on most sides, 4–6 minutes.
Transfer to a plate. They do not need to be cooked through yet.
Start the sauce. In the same pan, reduce heat to medium-low and add 1 tbsp olive oil if the pan is dry. Sauté the onion with a pinch of salt until soft and translucent, about 5 minutes.
Add the garlic and cook 30–60 seconds until fragrant.
Bloom the tomato paste. Stir in the tomato paste and cook 1–2 minutes, letting it darken slightly. This deepens the sauce’s flavor.
Add tomatoes and season. Pour in the crushed tomatoes and broth. Add oregano, sugar or balsamic, salt, and black pepper.
Stir and bring to a gentle simmer.
Simmer the meatballs. Nestle the meatballs into the sauce. Cover and cook at a low simmer for 15–20 minutes, turning once, until the meatballs reach an internal temperature of 165°F (74°C).
Finish the sauce. Stir in the torn basil. Taste and adjust salt, pepper, and acidity.
If the sauce is too thick, add a splash of broth or water; too thin, simmer uncovered a few minutes.
Serve. Spoon meatballs and marinara over whole-grain pasta or zucchini noodles. Top with a sprinkle of Parmesan and extra basil.