Boil the potatoes in unseasoned water until completely cooked through -reserved 2 tbsp of the cooking water. Drain them and leave to steam dry in the colander for a few minutes, then mash them thoroughly with the butter and laver (if using).
When the dough is stretchy and bouncy, move it to a greased bowl, cover with cling-film and leave it to rise somewhere warm until doubled in size and preheat the oven to 200°C.
MEANWHILE, blanche the leeks in seasoned boiling water until tender to the point of a knife. Quarter them lengthways into strips.
Whisk together the mustard, vinegar, oil, salt and pepper until you've got a thick sauce. Add the capers. Use this to dress the leeks in a small bowl. Check the seasoning.
In a bowl fold the brown crab meat through the mayonnaise and add the chives. Again, season to taste.
- Meanwhile, whisk together the flour with the sugar, salt, yeast and charcoal until combined. Add the egg and the (still warm) potato and get kneading -the dough you're looking for is slightly sticky and, rather excitingly, jet black.
When doubled, knock the dough back and divide into 4 equal pieces. Roll these into balls and place on a lined baking tray. Allow to rise for 15 minutes before brushing lightly with the egg wash. Sprinkle with flaked sea salt and bake for 20 minutes. Set aside to cool.
- Cut the burger buns in half and spread with the crab mayonnaise. Top with a tangle of the leeks then the cooked fish fingers. Place the pickled red onion rings on top followed by the final bun.
- Cook the Fish Fingers to packet instructions.