Heat 1 tablespoon oil in a large, high-sided skillet or Dutch oven over medium-high heat.
Add 1 pound chicken; cook, stirring often to break up the meat, until cooked through and beginning to brown, 6 to 8 minutes.
Add 1 cup onion, 1 cup tomatoes, 2 teaspoons Italian seasoning and ¼ teaspoon salt; cook, stirring often, until the onion is softened and the tomatoes are breaking down, 4 to 5 minutes.
Stir in 8 ounces pasta, 2 cups broth, 1 cup marinara and ½ cup cream; bring to a simmer over medium-high heat.
Cover and simmer, stirring occasionally, until the pasta is al dente, 10 to 12 minutes.
Stir in 5 ounces spinach (in batches, if necessary), ½ cup basil and ½ cup mozzarella; cover and cook until the spinach wilts and the cheese melts, about 2 minutes.
Garnish with more basil and 6 tablespoons Parmesan, if using.
