1. Bring a pot of salted water to a boil. In a serving bowl, whisk vinegar, mustard, honey and oil. Stir in onion; marinate for at least 15 minutes.
2. Have a large bowl of ice water ready. Blanch green beans in boiling salted water, about 3 minutes. Drain beans and immediately add to bowl with ice water to stop cooking. Drain beans again and pat dry. Cut into thirds crosswise.
3. Add green beans and remaining ingredients to bowl and toss to combine. Season with salt and pepper.