Peel the prawns if necessary.
Cut the lemon into quarters, and set aside.
Bring a large pot of water to a gentle simmer, add the quartered lemon and the salt.
Add the prawns to the simmering water, and remove the pot from the heat. The prawns should poach gently without overcooking this way.
Allow to cool down to room temperature.
Shred the baby gem lettuce into strips of about 1cm wide. Wash the leaves and spin dry, if necessary.
Trim the dried top of the spring onions, and slice into thin slivers.
Crush and peel the clove of garlic, and set aside.
Zest and juice the lemon, then toss with the baby gem, spring onion, garlic clove, salt and pepper.
Set aside until needed.
Zest and juice the lemon.
Whisk together the egg yolk, Dijon mustard, cayenne and lemon juice.
While whisking continuously, drizzle the sunflower oil until you obtain a thick mayonnaise.
Season with salt and pepper, Worcestershire sauce, cognac and ketchup.
Mix the cooled prawns with the Marie Rose sauce.
Fill the bottom of a tall glass with the shredded lettuce and spring onion. Top with the prawn salad.
Finish the dish with parsley just before serving.
«Every part of this dish can be made well in advance, however, it should be assemled at the last minute, otherwise the baby gem may go soft. Keep everything in the refrigerator, and put together when needed.»