Most reduced-fat recipes for fettuccine alfredo involve using low-fat alternatives like milk, roux and low-fat sour cream in place of high-fat ingredients like heavy cream, cream cheese, eggs and regular sour cream. Parmesan cheese is usually not substituted because its distinctive flavor is difficult to replicate.
For a recipe that uses a roux instead of heavy cream, start with a large saucepan on medium heat. In the saucepan, melt 1 teaspoon of unsalted butter until foaming, then whisk in 2 teaspoons of flour for one to two minutes, or until the mixture is smooth and golden. Whisk in 3/4 cup whole milk, 1/4 cup half-and-half, 1/2 teaspoon of salt, 1/2 teaspoon black pepper, 1 peeled and crushed clove of garlic, and a pinch of nutmeg.
Bring to a simmer, reduce heat to medium-low, and continue to simmer for one to two minutes until the slightly thickens. Discard the garlic, and stir in 1 cup of grated Parmesan cheese. Remove from heat.
Stir 9 ounces of fresh fettuccine pasta and 2 teaspoons of salt into 4 cups of boiling water. Stirring constantly, cook the pasta for two to three minutes, or until al dente. Drain the pasta, reserving 1 cup of the cooking water.
Return the sauce to low heat, add 1/3 cup of the cooking water and the cooked pasta, and toss to evenly coat the pasta. Cook for about one minute, until the sauce slightly thickens. Add more water to thin the sauce to the desired consistency and season to taste.
Serve immediately in a warmed bowl.