Heat olive oil in a large Dutch oven or heavy-bottomed pot over medium-high heat.
Add the Italian sausage and cook, breaking it into small crumbles with a wooden spoon, until browned and no longer pink (approximately 6-8 minutes).
Add the diced onion and sauté for 3-4 minutes until translucent.
Incorporate the minced garlic, tomato paste, and red pepper flakes; cook for 1 minute until the tomato paste darkens slightly and the garlic is fragrant.
Pour in the chicken broth and deglaze the bottom of the pot, scraping up all browned bits (fond).
Add the dried rigatoni and bring the mixture to a rolling boil.
Reduce heat to medium-low and simmer, uncovered, for 12-14 minutes, stirring occasionally to prevent sticking, until the pasta is al dente and the liquid has reduced by two-thirds.
Stir in the heavy cream and baby spinach; continue to cook for 2 minutes until the spinach has wilted and the sauce has thickened.
Remove from heat and fold in the grated Parmesan cheese until fully incorporated and creamy.
Adjust seasoning with salt and pepper before serving immediately.