For the trout, rub the lemon zest into the flesh of the trout. Place the cumin seeds, salt and freshly ground black pepper into a pestle and mortar and grind together until well combined. Reserve half a teaspoon for the vinaigrette and sprinkle the rest over the fish.
Heat the olive oil in a griddle pan and add the trout, spring onions and tomato half. Griddle the trout for 2-3 minutes on both sides, or until golden-brown and cooked through.
For the vinaigrette, place the soy sauce, lemon zest and juice and the reserved cumin mixture into a small bowl and whisk well to combine.
For the couscous, pour the stock over the couscous in a bowl and allow to soak for 5-7 minutes, or until all of the liquid is absorbed. Fluff the grains up with a fork and stir in the rest of the couscous ingredients.
To serve, spoon the couscous onto a serving plate and arrange the trout on top. Place the grilled spring onions and tomato half to the side, drizzle over the vinaigrette and serve.