Cut the bread into 8 slices and remove the crusts.
Put the mayonnaise in a bowl and add the crème fraîche, lemon zest and juice, cayenne pepper and tomato ketchup. Mix together well, then add the prawns and stir to combine.
Divide the prawn mixture between 4 of the bread slices then top with the remaining four slices of bread.
To serve, cut each sandwich into 3.
Chef's tips: to make it healthier, use half-fat crème fraîche. To make it cheaper, chop up a pack of seafood sticks and use in place of the prawns. To make it easier, use ready-made prawn cocktail filling, and to make it more indulgent, use cooked jumbo king prawns.