In a large pot or deep skillet, heat the olive oil 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 vegetable broth, diced tomatoes with their juice, dried penne pasta, dried oregano, crushed red pepper flakes, salt, and black pepper.
Stir to combine all ingredients.
Bring the mixture to a boil over high heat.
Once boiling, reduce heat to medium-low and simmer uncovered, stirring occasionally, for 12-15 minutes or until the pasta is cooked al dente and most of the liquid has been absorbed.
Add the fresh baby spinach leaves to the pot and stir until the spinach wilts, about 2 minutes.
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 desired.
Serve immediately.