To make the sauce, mix all ingredients in a bowl, season well and set to one side. If you’re making this the day before, cover and place in the fridge until needed. Peel, then chop the avocados into small chunks, tossing in lemon juice as you go, so they don’t discolour.
Mix the prawns with half the sauce, add the avocados and stir gently to coat everything. Heat a griddle pan. To make the crostini, rub a little olive oil onto each slice of ciabatta and grill on each side for 2 mins until toasted.
To serve, spread a little cocktail sauce on each crostini, top with a lettuce leaf and add a spoonful of the prawn cocktail mix. Sprinkle with a little cayenne pepper and serve with lemon wedges.