Start by opening the mackerel with a knife, cutting it lengthways from the tip of the head to the tail, leaving them butterflied.
After, rinse them in cold running water removing the gills and the bloodline- if you don’t, there will be a bitter taste to the fish.
Leave them facing down on a grill tray for 2 hours in the fridge. This will allow them to drain any excess water.
Wet your fingers and spread the Kombu sheets on a dry surface. Cut them so they can fit inside of the mackerel. Place the seaweed inside of the fish and leave overnight in the fridge to give flavour to the mackerel.
Cut the butter and dulce seaweed into small squares. Take the leftover kombu seaweed and blend together until smooth, then freeze.
Remove any bruised leaves from the cabbage. Cut as thin as possible.
In a pot of boiling water, add a pinch of salt and blanch the cabbage for 1 minute, then place the cabbage in ice water to chill.
Leave the blanched cabbage in a colander for 10 minutes to drain off any excess water, alternatively, gently squeeze the cabbage with your hands to remove excess water (be careful not to damage the leaves). If you have a colander leave the blanched cabbage there for 10 minutes to drain the excess water, or alternatively squeeze it with your hands without smashing it.
Light your wood fire grill and add some wood chips.
Remove the seaweed butter from the freezer and put it on a tray alongside some smoking wood chips.
Cover with some aluminium foil, then leave the tray covered for 10 minutes. Check the flavour of the butter for smokiness, if it needs more smoke, add some new wood chips and repeat the process.
Once you have a good fire going, remove the kombu sheet from the mackerels and grill them skin side down until they are golden brown. (it shouldn’t be necessary to grill from both sides).
Meanwhile squeeze the lemons, passing the juice through a sieve.
In a hot pan, add some olive oil and sauté the cabbage. Add a pinch of water, then slowly add pieces of butter to taste, stirring to make the mixture smooth. Check the seasoning with lemon juice and salt.
Serve with the grilled mackerel.