In a large pot, heat the olive oil over medium heat. Add the diced onion and sauté for about 3-4 minutes until translucent. Stir in the minced garlic and cook for an additional minute until fragrant.
Add the diced tomatoes (with their juices) to the pot and bring to a simmer.
Stir in the heavy cream and Italian seasoning. Season with salt and pepper to taste.
Add the pasta to the pot and stir to combine. Pour in enough water to just cover the pasta (about 2 cups). Bring to a boil, then reduce heat to low and cover. Cook for 10-12 minutes, stirring occasionally, until the pasta is al dente and the sauce has thickened.
Once the pasta is cooked, stir in the chopped spinach and grated Parmesan cheese until the spinach wilts and the cheese is melted.
Serve immediately, garnished with fresh basil leaves if desired.