Heat olive oil in a large deep skillet or Dutch oven over medium heat.
Add the Italian sausage, breaking it up with a spoon, and cook until browned and cooked through, about 5-7 minutes.
Add the diced onion and cook until softened, about 3 minutes.
Stir in the minced garlic and cook for 1 minute until fragrant.
Add the chopped sun-dried tomatoes, dried Italian seasoning, and crushed red pepper flakes; stir to combine.
Pour in the chicken broth and heavy cream, then add the penne pasta.
Stir everything together, making sure the pasta is submerged in the liquid.
Bring the mixture to a boil, then reduce heat to medium-low and simmer uncovered, stirring occasionally, until the pasta is tender and most of the liquid is absorbed, about 12-15 minutes.
Add the fresh baby spinach and stir until wilted, about 2 minutes.
Remove from heat and stir in the grated Parmesan cheese.
Season with salt and black pepper to taste.
Serve immediately, garnished with additional Parmesan if desired.