A flexible dish which would work as a starter as well as a main course
Brush the mackerel fillets on the skin side with a little oil, then rub in with your fingertips and season.
Grate the beetroot on a coarse grater into a bowl. Add the olive oil to bind, then add the chilli and drizzle over the balsamic glaze to taste. Mix gently and season with salt and pepper.
Preheat the grill to high place the mackerel fillets, skin side up, under the grill for about 3 minutes, then turn over. Spread the flesh side with the Dijon mustard and return to the grill for a further 2 minutes.
Place a pile of the beetroot salad in the centre of each plate and top with the mackerel fillets. Serve immediately with the diced tomato and a sprig of dill to garnish.