1. To make the pickling liquid, mix together the mustard seeds, vinegar, sugar and salt with 25g water in a small saucepan. Bring to the boil and add the shallot. Take off the heat then leave to infuse and cool.
2. For the mackerel, remove the 2 fillets and carefully pin bone, removing as many of the bones as possible. Slice each fillet in half, lengthways, then leave at room temperature while you prepare the remaining ingredients.
3. Quarter the cucumber and remove the seeds. Cut into even sized cubes. Mix together the buttermilk and horseradish, season to taste and set aside.
4. Once the pickling liquid has cooled, slice the grapes and add to the mixture, then leave for 20-30 minutes to lightly pickle.
5. Season the mackerel fillets and rub with a little oil. Place under a preheated grill for 1-2 minutes skin side up, then turn and grill for 1 minute until the flesh is just cooked.
6. Place the horseradish buttermilk, cucumber, pickled grapes and dill on 2 plates and top with the mackerel.