1. In a 4- to 5-quart pan, bring 2 quarts water to a boil over high heat. Add beans and cook, uncovered, until tender-crisp, 5 to 8 minutes; drain and pour into a bowl.
2. Meanwhile, mix mustard with vinegar, maple syrup, and oil. Pour over hot beans and mix to coat well; sprinkle with onion. Season to taste with salt and pepper.