Twist off the crab claws and set aside. Lay each crab on its back and use your fingers to push away the legs and centre of the crab, then pull away the soft gills at the centre of the crab and discard. Use a heavy knife to cut the inside into 4 pieces and using a skewer, remove the flesh. Scoop out the brown meat from the centre, then use a rolling pin to tap open the legs and claws and remove the white meat.
Heat 4 tablespoons of the oil in a frying pan, add the shallot and leek and cook over a low heat for 10 minutes until softened and golden. Add the garlic and cook, stirring, for 30 seconds. Stir in the chilli flakes, tomatoes and any crab liquid and cook for a further 5 minutes until pulpy. Stir through the crabmeat and season well.
Spoon the mixture into 4 gratin dishes. Mix together the breadcrumbs, parsley and the remaining oil in a bowl, then scatter over the crabmeat. Cook under a preheated medium grill for 3–5 minutes until crisp and golden.