Bring a 4-quart pot of salted water to a boil. Boil the haricots verts until crisp-tender, about 4 minutes. Drain in a colander and cool under cold running water; pat dry. Whisk the vinegar, mustard, and a pinch each of salt and pepper in a large bowl. Whisk in the oil until emulsified. Add the haricots verts, pecans, cranberries, and blue cheese and gently toss to combine. Arrange the lettuce leaves on a serving platter or individual salad plates and lightly season each with salt and pepper. Divide the salad among the lettuce leaves and serve.
