For the black pepper-crusted mackerel, heat the grill to hot. Brush the mackerel fillets with the mustard and roll the fillets in the black pepper.
Place on a baking tray and put under the grill for 3-4 minutes.
For the celery salad, put all the ingredients in a large bowl and mix together.
For the dressing, put all the dressing ingredients in a small bowl and whisk.
Just before serving pour the dressing over the salad
To serve, place the mackerel fillets on each of four plates with the salad alongside.