Make the meatball mix. In a large bowl, combine ground turkey, breadcrumbs, egg, Parmesan, grated onion, garlic, parsley, Italian seasoning, chili flakes, salt, and pepper.
Mix gently with a fork or your hands until just combined. Don’t overmix.
Form the meatballs. Roll into 1 to 1.5-inch balls. You should get about 18–20.
If the mixture feels sticky, wet your hands lightly with water.
Sear the meatballs. Heat 2 tbsp olive oil in a large skillet over medium heat. Add meatballs in a single layer. Sear 3–4 minutes per side until browned.
Transfer to a plate. They don’t need to be fully cooked yet.
Start the pasta. Bring a large pot of salted water to a boil. Cook pasta to al dente according to package directions.
Reserve 1/2 cup pasta water before draining.
Build the sauce. In the same skillet used for meatballs, add 1 tbsp olive oil if the pan looks dry. Add sliced garlic and chili flakes. Cook 30–60 seconds until fragrant.
Pour in crushed tomatoes, sugar (if using), and Italian seasoning. Season with salt and pepper.
Simmer with meatballs. Nestle the browned meatballs into the sauce. Cover and simmer on low for 8–10 minutes, until the meatballs are cooked through (internal temp 165°F/74°C).
Stir gently a couple of times.
Finish the sauce. Stir in a splash of reserved pasta water and 1/4 cup grated Parmesan. The sauce should look glossy and clingy. Add spinach now if using; stir until wilted.
Toss with pasta. Add drained pasta to the skillet.
Toss gently to coat, adding more pasta water as needed for a silky texture. Taste and adjust salt, pepper, and chili flakes.
Garnish and serve. Sprinkle with fresh basil and extra Parmesan. Divide into bowls and serve warm.