Cut the mackerel fillets in half then rub the skin side with a little oil and season.
Grate the beetroot on a course grater into a bowl, add one tablespoon of olive oil to bind, then add the chopped dill and balsamic to taste. Season with salt and pepper.
Place the mackerel fillets skin side up under a hot grill for about three minutes, then turn over. Spread the flesh side with a generous layer of Dijon mustard and return to the grill for a further two minutes.
Place a pile of the beetroot salad in the centre of your plate and add the fish fillets on top. Serve immediately with the diced tomato and garnish with a sprig of dill.