Cook onions in boiling water to cover 10 minutes; drain. Plunge into ice water to stop the cooking process; drain. Peel; set aside.
Cook butter and brown sugar in a saucepan over medium heat, whisking often, until butter melts. Whisk in vinegar and salt.
Add onions; bring to a boil. Reduce heat, and simmer, uncovered, 15 minutes or until thickened.