In 10-inch skillet, melt butter over medium- high heat. Add chicken; cook about 3 minutes, stirring occasionally, until outside turns white.
Stir in milk, hot water and sauce mix (from Chicken Helper box). Heat to boiling, stirring occasionally. Stir in uncooked pasta (from Chicken Helper box). Reduce heat; cover and simmer 9 to 11 minutes, stirring occasionally, until pasta is tender.
Stir in dressing and sour cream; sprinkle with onions. Remove from heat. Let stand uncovered 5 minutes (sauce will thicken as it stands).