COOK fettuccine according to package directions. Drain and return to pot.
PREPARE sauce while pasta is cooking. Melt butter over medium heat, add garlic and sauté for 1 minute. Stir in flour then whisk in milk until smooth. Cook and stir about 5 minutes until mixture is thick and bubbly.
REDUCE heat to low, stir in cheese, salt and pepper. Cook 1 to 2 minutes until cheese is melted. Add chicken, sour cream and peas and cook until heated through. Pour sauce over noodles and toss to coat. Sprinkle with parsley and top with extra pepper, if desired.