1. Place the lentils with the vegetable stock in a saucepan, bring to the boil, then simmer for 25–30 minutes, or until the lentils are just tender. Drain thoroughly and tip into a bowl.
2. Lay the mackerel fillets in a shallow dish, pour over the lemon juice and sprinkle with the lemon zest, and leave to marinade for about 5 minutes.
3. Stir the grated beetroot, parsley, dill and onion into the bowl of lentils with the vinegar and chilli flakes, then set aside.
4. Heat a large griddle pan or non-stick frying pan over a high heat and cook the mackerel for 2 minutes on each side, or until cooked through – you might need to do this in batches.
5. To serve, spoon the lentil salad onto plates and top with the griddled mackerel fillets sprinkled with sumac and a wedge of lemon on the side, if you wish.