Place the cous cous into a bowl and cover with warm water to just 1cm over the cous cous level. Mix in ½ tsp salt and set aside for 15 minutes to ‘swell’. Meanwhile heat the oil in a small frying pan and cook the onion over a moderate heat for 2-3 minutes until soft and just beginning to golden. Add the swollen cous cous and toss well to coat in the oil and mix with the onions. Add the lemon rind and mint and toss well together. Season generously with freshly ground black pepper. Preheat the oven to Gas 6/200°C/Fan 180°C.
Place 4 sea bass fillets onto a board, skin side down, flesh side up and divide the cous cous between the four fillets, compacting well into the centre of each. Place a second fillet skin side up, flesh side down on top of each and using some string tie the fillets together enclosing the filling. Place into a lightly buttered or oiled ovenproof dish and bake in the preheated oven for 20 minutes until the fish is opaque and cooked through.
In a small jug mix the lemon juice, olive oil and Dijon mustard together and season. Drizzle over the rocket and toss well. Serve piled on top of the baked fish. Serve with lemon wedges.
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