Cook pasta according to package directions; drain and keep warm. Meanwhile, in large saucepan, melt butter over low heat. Add garlic; cook and stir 30 seconds. Blend in flour; cook and stir 1 minute. Gradually whisk in milk. Cook and stir until mixture comes to a boil and thickens. Remove from heat; stir in cheeses and parsley until cheeses melt. Toss with fettuccini.
This is almost the way I make my sauce, but I use fat free half and half for the milk and omit the cream. Makes it so creamy you won't believe it's "light".