In 10-inch skillet, melt butter over medium-high heat. Cook chicken in butter about 3 minutes, stirring occasionally, until outside turns white.
Stir in milk, hot water, mushrooms 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 about 10 minutes, stirring occasionally, until pasta is tender.
Stir in spinach. Cook over medium-high heat about 2 minutes, stirring frequently, until hot. Remove from heat; uncover and let stand 5 minutes (sauce will thicken as it stands). Sprinkle with cheese.