Heat olive oil in a large deep skillet or pot over medium heat.
Add the chopped onion and sauté for 3-4 minutes until translucent.
Add the minced garlic and cook for 1 minute until fragrant.
Pour in the diced tomatoes with their juice, vegetable broth, and add the dried Italian seasoning, salt, and black pepper. Stir to combine.
Add the uncooked penne pasta to the pot and stir to submerge the pasta in the liquid.
Bring the mixture to a boil, then reduce heat to medium-low and simmer uncovered for 12 minutes, stirring occasionally to prevent sticking.
After 12 minutes, add the fresh baby spinach leaves on top of the pasta. Cover the pot and cook for an additional 2-3 minutes until the spinach is wilted and pasta is tender.
Remove the pot from heat and stir in the grated Parmesan cheese until melted and well combined.
Taste and adjust seasoning with additional salt and pepper if needed.
Serve hot, garnished with extra Parmesan cheese if desired.