Rice vinegar, sugar and salt are the keys to this quick and simple brine.
Combine first 5 ingredients in a small saucepan over medium-high heat; bring to a boil. Remove pan from heat; let stand 20 minutes or until cool.
Combine onion slices, cucumber slices, and dill sprigs in a large zip-top plastic bag; add vinegar mixture, and seal bag. Lay bag flat in refrigerator for 1 hour, turning bag occasionally.