01 - Bring a large pot of salted water to a boil. Cook the spaghetti according to package directions until al dente. Reserve 1/2 cup of the pasta cooking water, then drain the pasta thoroughly.
02 - While the pasta cooks, melt the butter in a large skillet over medium heat. Add the minced garlic and sauté for 1 to 2 minutes until fragrant and lightly golden, taking care not to brown or burn the garlic.
03 - Pour the heavy cream into the skillet and bring to a gentle simmer. Gradually stir in the Parmesan and mozzarella cheeses, whisking continuously until completely melted and the sauce becomes smooth and velvety.
04 - Add the salt, black pepper, and crushed red pepper flakes if desired. Stir well to incorporate the seasonings evenly throughout the sauce.
05 - Add the cooked spaghetti to the skillet with the sauce. Toss thoroughly to coat each strand with the creamy mixture. Add the reserved pasta water a tablespoon at a time as needed to achieve your preferred sauce consistency.
06 - Remove the skillet from heat. Sprinkle the chopped parsley generously over the top and add extra grated Parmesan. Serve immediately while hot and creamy.