In a large pot over medium heat, sauté the onion and garlic in the olive oil for 2-5 minutes.
Add in the tomato paste, salt, and pepper. Stir frequently for 1 minute.
Add in the broth, canned tomatoes, rotini, and cooked meatballs. Bring to a simmer over medium-high heat. Reduce heat to low and continue to simmer, covered, for 12-15 minutes until pasta is cooked al dente.
Stir in the baby spinach, heavy cream, and Parmesan until the spinach starts to wilt.
Serve and top with additional Parmesan if desired.