Bring onions, sugar, vinegar, bay leaf, kosher salt, mustard seeds, coriander seeds, and freshly ground pepper to a boil in a heavy saucepan over high heat, stirring often. Reduce heat to medium-high, and simmer, stirring often, 30 minutes or until liquid is reduced by half and mixture reaches a syrup-like consistency. Discard bay leaf. Cool to room temperature (about 45 minutes) before serving.
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