1.
Finely dice spring onions. Cut lemon in half. Cut one half into thin slices and squeeze juice from other half.
2.
Whisk oil and vinegar until combined; add salt and pepper to taste. Add spring onions and prawns. Marinate while making dressing.
3.
To make dill dressing, whisk sour cream and 1 teaspoon lemon juice in a bowl until creamy. Add salt and pepper to taste. Stir in dill.
4.
Cut avocados in half and remove stones. Remove flesh, leaving a rim about 5 mm wide. Dice flesh finely and mix with 1 tablespoon lemon juice and a little salt.
5.
Combine avocado, watercress, prawns and spring onions and spoon into avocado shells. Spoon dill dressing on the top. Garnish with lemon slices.