In a large pot or deep skillet, heat the olive oil over medium heat.
Add the chopped onion and cook 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 spaghetti. Make sure the pasta is mostly submerged in the liquid.
Add salt, black pepper, and red pepper flakes to the pot.
Bring the mixture to a boil, then reduce heat to medium-low and simmer, stirring occasionally to prevent sticking, for about 10 minutes or until the pasta is al dente and most of the liquid is absorbed.
Stir in the fresh baby spinach leaves and cook for 2 minutes until wilted.
Add the lemon zest, lemon juice, and heavy cream to the pot. Stir well to combine and heat through for 1-2 minutes.
Remove the pot from heat and stir in the grated Parmesan cheese until melted and creamy.
Taste and adjust seasoning with additional salt and pepper if needed.
Serve immediately, garnished with extra Parmesan if desired.