Rinse the fish, pat dry with kitchen paper and arrange in a shallow glass dish. Scatter the thyme leaves and lemon zest over the fish.
Mix 2 tablespoons of olive oil with the lemon juice and drizzle over the fish. Leave to marinate for about 5 minutes.
Meanwhile, cook the couscous according to packet instructions. Wash and halve the tomatoes and rinse the watercress. Fluff up the couscous grains with a fork, stir in the vegetables and a little salt and drizzle with the remaining tablespoon of olive oil.
Heat a large frying pan. Place the fish in the pan, drizzle with the marinade and cook on a medium heat for about 2-3 minutes on each side.
Divide the couscous between two plates and top with two mackerel fillets. Drizzle the pan juices over the fish and serve.
A quick, satisfying and so healthy meal with Omega 3 fats from the fish and antioxidants from the herbs and veg.