And here is their recipe.
Crispy mackerel sandwich
• cornish line caught mackerel
• sweet pickled red onions,
• watercress,
• fresh tartare sauce
• a nice lightly toasted bun of your choice (something quite light)
For the pickled red onions
Thinly slice your red onions with a mandolin and gently simmer for 20 minutes in red wine vinegar and a good helping of sugar. You are looking for the vinegar to be as sweet as it is sour to excite the palette. Remove from the hob and leave to cool. Once cool this can be kept in a Kilner jar in the fridge. The flavour will get even better with time.
For the tartare sauce
Of course you can buy a tartare sauce but we choose to make ours fresh for every event. Here is a quick version for you to try: mix mayonnaise with sliced capers, gherkins, grated hard boiled eggs, a finely sliced shallot and fresh chopped parsley in a bowl. Add lemon juice, sea salt and ground black pepper to taste.
For the mackerel
Rub a little oil on the skin of the de-boned mackerel and place on a hot griddle. Lightly toast the inside of the bread leaving the outside as it is. Ensure the underside of the mackerel is cooked well enough before turning to keep the fish in one piece and guarantee a crispy base - you are looking at around 60% of the cooking time on the underside. Turn over and allow to finish cooking through. Whilst waiting mix your watercress with a good pinch of the pickled red onions and allow the red wine vinegar to coat the watercress.
Make a bed out of the watercress and pickled red onions on the lightly toasted bread, lay the crispy mackerel on top and finish with a good dollop of cold tartare sauce. Enjoy.