In 12-inch skillet, melt butter over medium heat. Cook onion and mushrooms in butter 8 to 10 minutes, stirring occasionally, until onions are tender and beginning to brown. Stir in sugar. Cook about 3 minutes, stirring occasionally, until vegetables are very brown.
Stir gravy and sherry into vegetables. Add chicken to skillet; spoon sauce over chicken.
Cover; cook 5 to 10 minutes, turning chicken after half the cooking time, until thoroughly heated. Sprinkle with parsley.