Clean, wash and pat dry the celery. Set aside some nice leaves for garnishing. Cut the sticks diagonally into thin slices. Wash the grapes, dab dry and pluck off the stalks.
Halve and seed the berries. Wash the apple, dab dry and cut into very fine slices. Chop walnuts. Mix celery, fruit and half of the walnuts. Mix sour cream, lemon juice and sugar.
Cut the avocados in half, remove the seeds. Put the salad mix into the fruit halves and onto the plate. Pour sour cream over the filling, sprinkle remaining walnuts over it. Serve garnished with celery leaves and orange slices.