Shrimp and Avocado Appetisers
Scoop the avocado flesh into a bowl. Add some of the lime juice, salt, white pepper, and half of the watercress and mix together. Mash to a purée with a fork and spoon into four glasses. Put to chill.
Mix together the prawns, celeriac and cucumber and stir in the mayonnaise and yoghurt. Season to taste with the remaining chopped watercress, salt and white pepper.
Spoon the prawn mixture over the avocado mixture and garnish with the watercress and a wedge of lime. Chill until ready to eat.