Photograph: Karen Thomas
Preheat the grill to high. Drain the crab meat on kitchen paper to mop up the excess liquid.
Whisk the mayonnaise, lemon juice, cayenne pepper and a pinch of salt together in a large bowl, then mix in the crab meat and season to taste.
Cut the bread rolls in half horizontally and spread the cut sides with the soft butter. Grill, cut-side up, for 1-2 minutes or until golden. If serving the rolls cold, leave them to cool at this stage before topping with the crab mixture.
Generously spread the toasted rolls with the crab mixture and sprinkle with a little chopped spring onion and cayenne pepper. Slice each roll in half diagonally to serve.
Get ahead: make the crab mixture the day before; chill. Butter the bread rolls and slice the spring onion an hour before serving.
Kitchen secret: feeling particularly decadent? Chopped cooked lobster meat works well in place of the crab.