Heat a griddle pan until smoking hot. Cook the merguez sausages on the hot griddle for 2-3 minutes on all sides, turning regularly, until the sausages are cooked through. Squeeze over a little of the lemon juice.
Place the couscous into a bowl with the lemon zest, the remaining lemon juice, the red onion and olive oil. Pour over enough boiling water to cover the couscous by 1cm/½in. Cover with cling film and set aside for five minutes, or until the liquid has been absorbed by the couscous. Fluff the couscous with a fork and season to taste with salt and freshly ground black pepper. Add the coriander and mix well.
For the mayonnaise, place the egg yolks, vinegar and mustard into the bowl of a food processor and blend until pale and creamy. With the motor running, pour in enough vegetable oil, in a steady stream, to form a thick mayonnaise.
For the harissa, place all the harissa ingredients into a clean food processor and blend to form a smooth paste.
For the harissa mayonnaise, add 1-2 spoonfuls of the harissa to the mayonnaise (or more, to taste). Stir until well combined.
Serve the merguez sausages in a bread roll or with the couscous, with a spoonful or two of the harissa mayonnaise alongside.