Remove the fillets and chop it into pieces with the size of a finger.
In order to take some of the onions’ acrid edge off, use this technique: toss onions with a generous sprinkling of salt. Wait a few minutes. Rub salt into onions and then rinse, drain and blot dry onions.
Put the sea bass fillet pieces in a container with apple cider vinegar and keep the container for 2 hours in the refrigerator to be “cooked”.
After 2 hours, take it out of the refrigerator and drain the apple cider vinegar.
Put mustard, lemon juice, olive oil and 3 tablespoons of apple cider vinegar into another bowl and mix them thoroughly until you have a sauce consistency.
Pour this sauce into the container filled with fish and keep it in the refrigerator for 2 more hours. (It will be even more delicious if you keep it in the fridge overnight).
