Fill a large pot with water, bring to a boil and generously salt. Add the spaghetti and cook according to package instructions (about 8-10 minutes for al dente). Reserve about ½ cup of the pasta water, then drain the pasta.
Meanwhile, heat the butter and olive oil in a skillet over medium heat. Add the minced garlic and sauté for 1-2 minutes until fragrant and lightly golden — be careful not to burn it.
Lower the heat to medium-low. Add the Parmesan and mozzarella to the skillet and stir until the cheeses begin to melt. Gradually pour in the reserved pasta water while stirring, to achieve a smooth, creamy sauce.
Add the drained spaghetti directly into the skillet with the sauce. Toss with tongs or a spatula so each strand is coated. Taste and adjust seasoning with salt and pepper as needed.
Serve immediately in bowls, garnish with chopped fresh parsley and a sprinkle of extra Parmesan. Enjoy right away while the sauce is silky and creamy.
Notes
If the sauce becomes too thick, add a splash more pasta water to loosen it. For a variation, you can add cooked shrimp, spinach or mushrooms. Leftovers can be stored in an airtight container in the fridge for 3-5 days or frozen up to 3 months.