Butter beans resemble lima beans, which can also be used in this salad.
Bring a large stockpot of salted water to a boil, and prepare an ice bath. Blanch the butter beans just until tender, about 4 minutes. Transfer to the ice bath, and cool. Drain in a colander.
Blanch the snap peas in the boiling water just until tender, about 2 minutes. Transfer peas to the ice bath, and cool. Drain in a colander. Peel the skins from the butter beans, and set beans aside.
Heat 2 tablespoons walnut oil in a large skillet over medium heat. Add the leeks, and cook, stirring occasionally, until bright green and soft, about 12 minutes. Add 1/2 cup water, and stir well. Cover, and cook until leeks are soft, about 3 minutes. Season with salt and pepper. Add the reserved beans and snap peas, and toss over low heat until hot throughout. Adjust seasoning.
Transfer mixture to a large serving bowl. Add the chervil, pea shoots, if using, and sorrel; toss. Drizzle with the remaining tablespoon walnut oil, or to taste, and serve.
Pea shoots are available in the spring at green markets or farm stands.