Cook sausages on a hot smoking griddle for 2-3 minutes per side until cooked through.
Squeeze lemon juice over the cooked sausages.
Mix couscous, lemon zest, remaining juice, onion, and olive oil in a bowl.
Cover couscous with boiling water by 1cm and set aside for five minutes.
Fluff couscous with a fork and season with salt, pepper, and fresh coriander.
Blend egg yolks, vinegar, and mustard in a food processor until creamy.
Slowly stream in vegetable oil with the motor running to thicken the mayonnaise.
Blend all harissa ingredients in a clean food processor into a smooth paste.
Stir 1-2 spoonfuls of the harissa paste into the mayonnaise.
Serve sausages with the prepared couscous and harissa mayonnaise.