Adjust two oven racks to the upper and lower thirds of the oven and preheat to 425 degrees F. Spread the walnuts on a baking sheet and the bread on another baking sheet. Bake the walnuts until golden and toasted, about 10 minutes. Bake the bread until lightly golden brown and crispy, 10 to 12 minutes. Set both aside to cool completely. Finely chop 2 cups of the walnuts and set aside.
Meanwhile, whisk together the sour cream, mayonnaise, lemon zest and juice, parsley, scallions, 1/2 teaspoon salt and a few grinds of pepper in a medium bowl. Set aside.
Put the chicken in a food processor along with 1 cup of the sour cream mixture, 1/4 teaspoon salt and a few grinds of pepper. Process until the mixture is a chunky puree. Transfer to another medium bowl and set aside.
Rinse out the food processor, and add the toasted bread, 2 cups of the unchopped walnuts, the mustard and 1 teaspoon salt. Process until the mixture resembles a chunky puree. With the motor running, slowly drizzle in the oil and 2 tablespoons water through the feed tube. Continue to process until smooth.
To assemble the dip, make an even layer of the chicken salad in the bottom of a 2-quart trifle bowl. Then top with even layers of the following: the walnut mixture, chopped celery stalks, blue cheese, the reserved finely chopped walnuts, the remaining sour cream-herb sauce and then the cranberries. Sprinkle with the celery leaves. Serve right away, or cover and refrigerate up to 8 hours or overnight. Serve with crackers.
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