In a large pot of boiling salted water, cook pasta according to package instructions; drain well.
Melt butter in a saucepan over medium heat. Whisk in flour until lightly browned, about 1 minute. Gradually whisk in milk, and cook, whisking constantly, until incorporated, about 1-2 minutes. Stir in heavy cream and garlic powder until slightly thickened, about 1 minute.
Stir in cheeses until melted, about 1-2 minutes. If the mixture is too thick, add more milk as needed; season with salt and pepper, to taste.
Stir in pasta and gently toss to combine.
Serve immediately, garnished with parsley, if desired.
1 cup fettuccine
2 tablespoons unsalted butter
2 tablespoons all-purpose flour
1 1/4 cups 2% milk, or more, as needed
2 tablespoons heavy cream
1/4 teaspoon garlic powder
1/4 cup Neufchatel cheese, cubed
1/4 cup freshly grated Parmesan cheese
Kosher salt and freshly ground black pepper, to taste