Cook the lasagne sheets in boiling salted water for 5-7 minutes and place in cold water. Wash the cucumber and slice finely. Peel onion and chop very finely. Clean, wash and drain the lettuce and cut into strips, except for a few leaves to lay out the plate.
Wash parsley, dab dry and chop finely. Peel garlic and press it through a garlic press. Mix vinegar, garlic, onions, parsley, salt, pepper and sugar. Add yoghurt, salad cream and oil and stir in.
Add the prawns, cucumber and salad to the sauce. Dab the lasagne sheets dry and arrange them on salad leaves, sprinkled with coloured pepper. Serve on a plate with lemon balm and lemon wheels.