Heat grill to high. Put the aubergine on a baking sheet, brush with oil and season. Grill for about 15 mins, turning and brushing with more oil halfway, until browned and softened.
Meanwhile, tip the couscous into a large bowl, pour over the stock, then cover and leave for 10 mins. Mix the tomatoes, mint, goat’s cheese and remaining oil together. Fluff the couscous up with a fork, then stir in the aubergines, tomato mixture and lemon juice.