Preheat the oven to 350°. Spread the walnuts in a pie plate and toast for about 10 minutes, until golden and fragrant. Let cool, then coarsely chop the nuts.
In a large bowl, whisk the yogurt with the mayonnaise, parsley, chives and tarragon and season with salt and pepper. Fold in the turkey, cranberries and walnuts.
In another bowl, toss the mixed greens with the oil and lemon juice; season with salt and pepper. Mound the salad on a platter and top with the turkey salad. Serve right away.