Heat 1 tbls butter in 10" skillet. Add mushrooms and saute over medium-high heat for 2 minutes. Turn mushrooms onto heated platter. Add remaining butter to skillet. Place chicken into pan. Saute over medium heat about 10 minutes, until chicken is cooked through, turning 2-3 times. Add onions and wine after 5 minutes of cooking. Arrange cooked chicken over mushrooms. Drain mushroom juice into skillet. Heat pan juices to boiling. Add cream. Boil 2-3 minutes. Add grapes to pan. Heat through. Stir in salt and pepper. Pour sauce and grapes over chicken. Serve immediately. (If chicken has cooled off, return to pan for a few minutes to reheat).
Can be made ahead, covered and refrigerated. Before serving, return to pan and reheat over low heat, being very careful not to brown. OR, reheat, covered, in microwave using MEDIUM power for 1 minute.