This dish is bursting with omega-3, thanks to the mackerel which is packed with these essential fats.
Score the flat surface of the sweet potato slices and arrange them in a roasting tin in a single layer.
Slash the mackerel fillets 2–3 times on each side then arrange on a large sheet of aluminium foil in a separate roasting tin.
Mix together the marinade ingredients and brush over the sweet potato and mackerel. Leave the sweet potato slices to marinate for 5-10 minutes.
Preheat the oven to 190°C/fan 170°C/gas mark 5. Roast the sweet potatoes for 40 minutes in the marinade. Add the tray of mackerel for the last 8 minutes of the cooking time.
Make the salad: toss the cucumber together with the vinegar, sugar, salad leaves and herbs. Divide between 4 plates, top with the sweet potato slices and mackerel fillets, sprinkle over the sushi ginger and serve straight away.