Heat olive oil in a large skillet over medium-high heat. Add the Italian sausage and cook, breaking it apart with a spoon, until browned (5-7 minutes). Add the chopped onion and cook until softened (3-4 minutes). Stir in the minced garlic and red pepper flakes and cook until fragrant, about 1 minute.

Pour in the chicken broth and heavy cream, scraping any browned bits from the bottom of the pan. Bring to a simmer. Stir in the dry penne pasta. Reduce heat to medium-low, cover, and cook for 12-15 minutes, stirring occasionally, until pasta is al dente and has absorbed most of the liquid.

Turn off the heat. Add the fresh spinach and stir until it wilts. Sprinkle in the Parmesan cheese and stir until melted and the sauce is creamy. Season to taste with salt and freshly cracked black pepper. Let rest for 2-3 minutes for the sauce to thicken.

Serve immediately, garnished with additional Parmesan cheese and cracked black pepper.