Plum-Glazed Duck

3 – 4 duck breasts, cleaned and rinsed
garlic salt
1/4 cup butter or margarine
1 medium onion, chopped
1 – 17oz can purple plums
1/2 cup packed light brown sugar
1/2 cup chili sauce
1/4 cup soy sauce
1/4 cup lemon juice
1 teaspoon ground ginger
2 teaspoons prepared mustard
1 teaspoon worchestershire