1) In 4 glasses, place the lettuce at the bottom.
2) Then place the prawns and season with salt and pepper to taste.
3) In a mixing bowl, season the egg yolk
4) Stir in the oil, a drop at the time, until thick and smooth.
5) Gradually, add vinegar until the mixture has a thick creamy consistency.
6) Stir in the tomato ketchup; blend thoroughly.
7) Spoon the mixture equally over the prawns in the glasses.
8) Garnish each Prawn Cocktail serving with a lemon slice and serve chilled
with thinly sliced brown bread.