In a large bowl, gently combine ground turkey, breadcrumbs, Parmesan, egg, garlic, Italian seasoning, salt, and pepper until just mixed.

Roll mixture into 1-inch meatballs, then heat olive oil in a skillet and brown on all sides until golden, about 5 minutes.

Add chopped onion and garlic to the same skillet; sauté until onion is translucent and garlic fragrant, about 3 minutes over medium heat.

Stir in crushed tomatoes and red pepper flakes; bring to a gentle simmer, then nestle meatballs back in; cover and cook 20 minutes.

Meanwhile, boil spaghetti in generously salted water until al dente, about 8–10 minutes; drain, reserving ½ cup pasta water.

Toss drained spaghetti into sauce; add reserved pasta water if needed, then garnish with fresh basil and extra Parmesan.