Bring a large pot of salted water to a boil. Add all of the beans and cook until crisp-tender, 3 to 5 minutes (they'll all be done at roughly the same time). Drain, transfer to a bowl of salted ice water to stop the cooking, and drain well. Transfer to a mixing bowl and toss int he shallots, walnuts, and parsley.
In a jar, combine the mustard, water, sugar, lemon juice, oil, creme 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.