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Heat the oil in a large non-stick frying pan, add the onions, fennel, garlic and cumin seeds and fry, stirring frequently, for about 15 mins until the vegetables are soft and golden. Stir in the lemon zest and season. Leave to cool.
Stir the harissa with the agave or honey. Place a salmon fillet, skinned-side down, on a large sheet of baking parchment and spread with half the harissa mixture. Top evenly with the vegetables, then cover with the other salmon fillet, positioning it so that the thickest end is on top of the thinnest end of the other fillet. Can be made up to this point, covered and chilled the day before.
Heat oven to 200C/180C fan/gas 6 with a large baking sheet inside. Brush the salmon with the rest of the harissa mixture, then lift (still on the parchment) onto the hot baking sheet and bake for 30 mins. Remove from the oven and check that the fish is cooked all the way through; if not done to your liking, cook 5 mins more and check again.
To make the couscous, crumble the stock cube into a large bowl and add the harissa, currants and couscous. Meanwhile, put the chickpeas in a bowl with the lemon juice, oil and seasoning. Just before serving, pour 450ml boiling water over the couscous, stir well, then cover with a plate and leave to soak for 5 mins. Add the lemon zest, parsley and mint (if using) to the chickpeas and stir well, then toss through the hot couscous with the flaked almonds. Serve on a big, warm platter with the salmon (the easiest way to transfer the salmon is to lift it still on the parchment, then slide it on top), with lemon wedges on the side. To serve, cut the salmon in slices, or cut thicker slices and halve to make chunky squares.