In a small bowl, toss onion with 1 teaspoon salt; let stand for 1 hour. Drain and rinse in cold water; set aside. , In a small saucepan, combine the vinegar, water, sugar, garlic, mustard seed, mustard, celery seed and remaining salt. Bring to a boil. Reduce heat; simmer, uncovered, for 1-2 minutes or until sugar is dissolved. Stir in onion; simmer 3 minutes longer or until onion is heated through, stirring often. , Transfer to a serving dish. Cool to room temperature. Cover and refrigerate for at least 3 hours before serving. Serve with a slotted spoon.