In 10-inch skillet, melt butter over medium heat. Cook chicken and bell pepper in butter, stirring occasionally, until outside of chicken turns white.
Stir in uncooked pasta and sauce mix (from Hamburger Helper box), water, tomatoes and mushrooms. Heat to boiling, stirring occasionally. Reduce heat; cover and simmer about 10 minutes, stirring occasionally, until pasta is tender. Remove from heat; sprinkle with cheese.
Cover; let stand 5 minutes (sauce will thicken as it stands).