Preheat oven to 375°F. Spread walnuts on a shallow baking sheet and roast in oven until toasted, about 10 minutes. Let cool.
Coarsely chop walnuts and transfer 3/4 cup of them to a bowl with chicken, grapes and celery; toss.
In a small bowl, whisk together scallions, mayonnaise, yogurt, vinegar, curry powder and pepper. Scrape mixture over chicken mixture and toss well.
To serve, arrange a radicchio cup in center of a plate. Fill radicchio with chicken salad. Arrange greens around radicchio. Sprinkle remaining nuts on top of salad.