Cook the spaghetti:
Bring a large pot of salted water to a boil. Add the spaghetti and cook according to the package directions until al dente. Save 1 cup of pasta water, then drain the spaghetti and set it aside.
Prepare the sauce:
In a large skillet, melt the butter over medium heat. Add the minced garlic and sauté for 1-2 minutes until fragrant.
Lower the heat to low and stir in the heavy cream and cream cheese. Whisk until the cream cheese has melted and the mixture is smooth.
Add the Parmesan cheese:
Stir in the Parmesan cheese, Italian seasoning, and red pepper flakes (if using). Cook for 2-3 minutes, stirring frequently, until the sauce thickens slightly.
Combine the pasta and sauce:
Add the cooked spaghetti to the skillet and toss to coat evenly with the sauce. If the sauce is too thick, gradually add the reserved pasta water, a few tablespoons at a time, until the desired consistency is reached. Season with salt and black pepper to taste.
Serve:
Transfer the spaghetti to serving plates or a large bowl. Garnish with fresh parsley and extra Parmesan cheese, if desired. Serve immediately.