Bring a large pot of salted water to a boil and cook the pasta until just al dente according to the package directions. Reserve about 1 cup of pasta water, then drain the pasta; the noodles will finish cooking briefly in the sauce.

Heat the olive oil in a large skillet over medium heat. Add the diced onion and cook for 3 to 5 minutes, stirring occasionally, until softened.

Add the ground turkey and break it into small pieces with a spoon. Cook until no longer pink and the thickest pieces reach 165°F, then season with salt, pepper, and Italian seasoning.

Stir in the garlic and red-pepper flakes, if using, and cook for about 30 seconds. Add the tomatoes and chicken broth, scraping up any browned bits from the skillet.

Simmer the turkey sauce for 8 to 10 minutes, or until slightly thickened. Add spinach or another quick-cooking vegetable during the final few minutes if desired.

Add the drained pasta to the skillet and toss until coated. Stir in reserved pasta water a little at a time if the sauce needs loosening, then finish with Parmesan, mozzarella, or fresh herbs.