Method:
Put chicken in a small saucepan and cover with cold water. Bring to a boil, lower heat and simmer until the chicken is just cooked through, 15 to 20 minutes. Drain (reserve the cooking liquid for another use if you like) and cool.
In a blender, combine cashews, grape juice, tamari and shallot and blend until smooth. Transfer to a small bowl and stir in tarragon. In a large bowl, combine lettuce, radicchio and onion and toss with about 2/3 of the dressing. Mound the mixture on a platter and top with the grapes. Dice the chicken, sprinkle it over the salad, and drizzle with the remaining dressing.