In a bowl, pour the juice of 2 lemons and add the sugar, cucumber ribbons and ½ of the dill.
Bring a large pan of water to the boil and add the asparagus. Blanch for 2 minutes and then set aside in a bowl of ice cold water.
Meanwhile, toast the sourdough slices on a griddle pan.
In a small bowl, combine the mayonnaise and horseradish sauce. Spread the mayonnaise mix on each slice of toast, and then slice them in half. Top each slice with a generous layer of prawns and then arrange some of the cucumber pickle.
Chop the asparagus into 5cm pieces and place a few pieces on top of each sandwich.
To serve, place the remaining prawns in jars. Then arrange the open sandwiches on a platter with the prawn jars and watercress on the side, and then sprinkle the beetroot cubes and remaining dill over.