In a small saucepan, combine vinegar, ½ cup (120 grams) water, sugar, salt, and coriander seeds (if using). Bring to a boil over medium-high heat; cook, stirring occasionally, for 2 minutes. Remove from heat, and pour over onion. Let cool for 30 minutes; refrigerate until ready to use. (Can be made 1 day ahead and refrigerated in an airtight container.)