As soon as chicken is in oven, in 10-inch skillet, heat oil and 1 tablespoon butter over medium heat until butter stops sizzling. Add onion, brown sugar and 1/4 teaspoon salt. Cook 20 minutes, stirring occasionally, until onion is soft and golden brown. (Do not rush by turning up the heat.) Remove onion from skillet, using slotted spoon; set onion aside.