1
Trim ends of beans and leave them whole. Drop beans into a saucepan with salted boiling water. Cook until crisp tender, about 3 to 5 minutes according to the size of the beans. Do not overcook. Drain and rinse well with cold water until cool.
2
In a salad bowl, add mustard, vinegar, onion, garlic, oil, salt, and pepper and blend with a wire whisk. Add beans and tomatoes, toss well, and sprinkle with chives.