Heat a griddle pan over a high heat until very hot. Brush the vegetables all over with the oil and griddle for 5 minutes, turning once, until charred and tender.
Meanwhile, preheat the grill to medium. Put the fish on a baking tray and brush each fillet with the harissa paste. Season with freshly ground pepper and cook under the grill for 4-5 minutes, until cooked.
Cook the couscous according to the instructions on the packet. Divide it among the plates and top with the griddled vegetables and harissa sea bass.