Cut the red onions into wedges, keeping the root ends intact to hold the wedges together.
Thread the garlic cloves and chillies on to separate metal skewers, then cook all the vegetables on a hot barbecue as follows, removing them when charred on all sides to a large serving bowl: aubergine, pepper and red onions for 5 minutes on each side; courgette for 4 minutes on each side; chillies for 2—3 minutes on each side and garlic for 1 minute on each side.
Place the couscous in a bowl and add enough hot water to cover. Leave for 5 minutes to allow all the water to be absorbed.
Peel the chillies and pepper when cool enough to handle, then deseed them. Roughly chop all the vegetables and add them to the bowl of couscous. Add the cumin, paprika, chilli flakes and seasoning, and mix well. Drizzle with the olive oil and garnish with the chopped coriander and lemon wedges, if using.