Bring 4 quarts of heavily salted water to a boil in a large pot.
Cook penne pasta until al dente, approximately 10 to 11 minutes; reserve 1/2 cup pasta water then drain.
Heat olive oil in a large skillet over medium heat.
Sauté onion for 5 minutes until translucent, then add garlic and cook for 60 seconds.
Add crushed tomatoes, oregano, and red pepper flakes to the skillet; simmer on low for 10 minutes.
Whisk in heavy cream and simmer for an additional 2 minutes.
Add spinach to the sauce and stir until wilted, approximately 1 to 2 minutes.
Toss the cooked pasta and Parmesan cheese into the skillet.
Adjust sauce consistency with reserved pasta water as needed.
Season with salt and pepper and serve immediately.