Heat half the oil in a large frying pan or wok, add the mushrooms and sauté for 3-4 mins until golden. Transfer to a plate and set aside.
Add the remaining oil, vegetables and garlic to the pan and sauté for 2-3 mins. Add the harissa, tomatoes and stock, then season to taste. Cover the pan, bring to the boil and simmer for 15-20 mins or until the squash is tender.
Stir in the mushrooms and simmer uncovered for a further 5 mins. Stir in the coriander and serve, then serve spooned over couscous.