Bring a pan of water to the simmer and place a bamboo steamer on top.
Arrange the lemon slices in the steamer and place the mackerel fillets inside. Season with salt and freshly ground black pepper. Steam for 3-4 minutes, or until cooked.
For the leeks, heat the red wine in a pan. Add the leeks. Poach for 4-5 minutes, until tender.
To serve, place the mackerel fillets on a serving plate and spoon the poached leeks alongside.