Put the crab and breadcrumbs into a large mixing bowl. Finely slice the spring onions, then chop the parsley and coriander leaves. Add all three to the crab and mix gently.
In a second bowl, break the eggs and beat lightly to mix yolks and whites, then blend in the mustard, harissa and mayonnaise. Stir into the crab gently combining crab, crumbs and seasoning.
Shape the crab mixture into 20 balls, each weighing approximately 45g, rolling them lightly in your hands, floured if necessary. Let the balls settle for 20 minutes in the fridge.
Pour enough oil into a pan in which to deep fry the croquettes and heat to 180C on a kitchen thermometer. Place one of the croquettes on a draining spoon and lower into the oil – it should crackle immediately – followed by 5 or more of the others, taking care not to crowd the pan. Keep an eye on the heat – you need it to remain fairly static.
Fry the croquettes for 2-3 minutes until golden, lift out and drain on kitchen paper then repeat with further batches until the mixture is finished. Serve hot, with halves of lemon or lime.