In 2-quart saucepan, heat water to boiling. Add beans. Cover and cook about 3 minutes or until bright green but still crisp; drain.
In medium bowl, toss beans, dill weed and onion.
In 1-quart saucepan, mix vinegar, oil, sugar, garlic and red pepper flakes. Heat to boiling; reduce heat. Simmer uncovered 2 minutes, stirring occasionally. Pour over green beans; mix well.
Cover and refrigerate 20 minutes or until serving.