Heat the oil in a large Dutch oven or saucepan over medium heat. Add the onion and cook, stirring frequently, until softened, about 7 minutes. Add the garlic and cook an additional minute.
Raise the heat to medium-high. Add the meat, Italian seasoning, pepper, and red pepper flakes, and cook, breaking up the large pieces until no longer pink, about 5 minutes. Drain any excess fat.
Stir in the broth, tomato sauce, 3/4 cup of the milk, and the spinach, cover, and bring to a boil.
Stir in the pasta and return to a boil. Reduce the heat and cook at a low boil, covered, stirring occasionally, until the pasta is tender, about 9 minutes.
In a bowl, whisk together the cornstarch and remaining 1/4 cup milk until well combined. Stir the cornstarch mixture into the pasta. Maintain at a low boil and stir gently until the mixture thickens, about 2 minutes. Remove from the heat and stir in the Parmesan cheese and fresh basil.
Top individual servings with additional Parmesan cheese, red pepper flakes, and chopped basil as desired.