In a large deep skillet or pot, heat 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 and water, then add the dried penne pasta.
Stir to combine and bring the mixture to a boil.
Reduce heat to medium-low and simmer uncovered, stirring occasionally, for 10 minutes or until the pasta is almost cooked and most of the liquid is absorbed.
Add the chopped spinach, salt, black pepper, and crushed red pepper flakes to the pot.
Stir in the heavy cream and grated Parmesan cheese until the spinach wilts and the sauce becomes creamy, about 2-3 minutes.
Taste and adjust seasoning if needed.
Serve immediately, garnished with extra Parmesan if desired.