Bring a large pan of water to the boil. Add the asparagus and boil for 2-3 minutes until just tender. Remove with tongs and place into ice water to prevent further cooking. Remove and leave to dry on kitchen towel-lined plates.
Next, roughly chop 150g prawns and add to a medium bowl with 150g cream cheese, nearly all the lemon juice and half the zest. Lightly season and use a stick blender to blend into a smooth paste. Add the double cream to a small bowl and whisk by hand until softly whipped. Fold into the prawn mixture along with the dill. Place in the fridge and leave to chill.
Slice off the domed top from one of the loaves and set aside. Trim the sides from the remaining loaf and carefully carve off the bottom crust. Trim off any excess bread so it’s as flat as possible. Repeat with the second cottage loaf. Take the reserved domed top and slice the top so that it’s flat. Trim off as much of the hard crust as possible so it’s roughly the same size and shape as the other bread layers.
Leaving the thinner bread layer until last, take one of the other layers and place on a cake stand or decorative plate. Dollop on some of the prawn mousse and spread evenly. Add half the cucumber slices in a single layer, followed by half the smoked salmon.
Take the second layer of bread and spread a thin amount of the prawn mousse on one side of the bread. Place spread side down on the salmon. Add another layer of prawn mousse followed by the remaining cucumber (reserving about 7 slices) and salmon. Take the final layer and spread with the last of the prawn mousse. Place it spread side down on top. You should now have something that resembles a three-layer cake.
Whisk the sour cream and remaining cream cheese together with the last of the lemon juice until smooth. Taking a palette knife, spread the cream cheese mixture over the entire cake until smooth and even all over. Arrange the asparagus around the outside of the cake so that the spears are standing up.
Add the remaining prawns to the top of the cake in a decorative pattern. Cut a snip into each remaining slice of cucumber. Fold the cucumber into itself so that the cut edges overlap. Push each piece down on to a toothpick and repeat with the other slices, sliding them on to the toothpick to resemble petals. Push the cucumber flower into the centre of the sandwich cake to serve.