1.Put chicken, onion, walnuts, celery, mayonnaise and sour cream into a large bowl, mix the ingredients together with a spoon.
2.Spread half the chicken mixture over 10 slices of white bread, put a slice of wholemeal bread on top of each. Spread the rest of the chicken mixture over the wholemeal bread, put the other 10 white bread slices on top.
3.Use a serrated bread knife to cut the crusts off the sandwiches, cut each sandwich into four triangles.
To save time chopping the walnuts, put the roasted nuts in a food processor and pulse a few times until the nuts are finely chopped but not ground.