In a large pot, combine the uncooked spaghetti, water, whole milk, minced garlic, unsalted butter, salt, black pepper, and crushed red pepper flakes.
Bring the mixture to a boil over medium-high heat, stirring frequently to prevent the pasta from sticking to the bottom of the pot.
Once boiling, reduce the heat to medium and continue to cook, stirring often, until the pasta is tender and most of the liquid is absorbed, about 10 to 12 minutes.
Remove the pot from heat and immediately stir in the shredded sharp cheddar cheese and grated Parmesan cheese until melted and creamy.
Taste and adjust seasoning with additional salt and pepper if desired.
Sprinkle chopped fresh parsley over the pasta before serving.