Combine onions and dill in a plastic bowl (do not use glass).
Combine vinegar and next 3 ingredients in a medium saucepan; bring to a boil over medium-high heat. Remove from heat. Pour hot vinegar mixture over onion rings and dill in bowl; cover and chill at least 8 hours. Store in an airtight container in the refrigerator up to 1 month. Serve with a slotted spoon.