Place prawns in a steamer over boiling water.
Cook prawns for 1.5 minutes, add langoustines, and cook another 1.5 minutes until pink.
Cool seafood on a plate for 10 minutes and then peel.
Blend egg yolks and mustard in a processor until creamy.
Slowly pour in oil with the motor running until mayonnaise thickens.
Mix mayonnaise with Worcestershire sauce, Tabasco, brandy, ketchup, seasoning, and lemon juice.
Slice the remaining lemon into four wedges.
Toss lettuce leaves with French dressing in a bowl.
Layer lettuce, seafood, and sauce in glasses, then garnish with cress, cayenne, and lemon.