Put the flour and egg on separate plates. Mix together the breadcrumbs, sumac (if using) and the lemon rind on another plate. Dip the aubergine in the flour and shake to remove any excess. Season well, then dip in the egg and finally in the breadcrumb mixture, making sure each slice is well coated.
Heat half the vegetable oil in a large, nonstick frying pan. Add half the aubergines and cook for 3–5 minutes, turning once, until golden and cooked through. Keep warm and repeat with the remaining aubergine and oil.
Meanwhile, place the couscous in a bowl and pour over the stock. Cover and leave for 5 minutes until all the liquid has been absorbed. Pour over the lemon juice and olive oil and leave to cool a little. Stir through the cucumber, spring onion and herbs and serve alongside the aubergine.