Fast and easy pickled red onions instantly upgrade just about any dish. We love them on Martha's favorite Hake Burgers, but try adding them to eggs, tacos, grilled meats, and salads.
Place onions in a medium heatproof bowl. Bring vinegar, sugar, and salt to a boil in a saucepan; pour over onions. Let cool completely. Refrigerate, covered, at least 2 hours and up to 1 month.