1 1/2 shallots, finely chopped
1/2 cup walnut halves and pieces, toasted (toast walnuts at 375 on a baking sheet for 7 to 8 minutes)
1 handful of fresh flat-leaf parsley, chopped
1 Tb grainy mustard
1 Tb hot water
1/2 tsp sugar
Juice of one lemon
1/3 cup extra-virgin olive oil
1/4 cup crème fraiche or sour cream
Kosher salt and freshly ground pepper
Bring a large pot of salted water to a boil. Add all of the beans and cook until crisp-tender, 3 to 5 minutes. Drain, transfer to a bowl of salted ice water to stop the cooking, and drain well. Transfer to a mixing bowl and toss in the shallots, walnuts, and parsley.
In a jar, combine the mustard, water, sugar, lemon juice, oil, crème fraiche, salt, and pepper. Put the cap on and shake vigorously to emulsify. Just before you serve the salad, pour the dressing over the beans and toss well to coat with the dressing.
Source: Eat this Book