Heat olive oil in a large deep skillet or Dutch oven over medium heat.
Add the Italian sausage, breaking it apart 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 diced tomatoes with their juices, chicken broth, dried Italian seasoning, crushed red pepper flakes, salt, and black pepper. Stir to combine.
Bring the mixture to a boil.
Add the uncooked penne pasta to the pot and stir to submerge the pasta in the liquid.
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.
Stir in the fresh baby spinach leaves and cook until wilted, about 2 minutes.
Remove the pot from heat and stir in the grated Parmesan cheese until melted and well combined.
Serve hot, garnished with additional Parmesan if desired.