Melt remaining butter in medium saucepan over medium heat. Add onion; cook, stirring occasionally, for 1 to 2 minutes or until tender. Stir in flour, salt and pepper; cook, stirring constantly, for 1 minute. Remove from heat; gradually stir in broth. Return to heat; bring to boil over medium heat, stirring constantly. Stir in evaporated milk and remaining cheese; cook over low heat until cheese melts. Remove from heat. Stir in chicken, mushrooms, peas and sherry.