In large heavy saucepan over medium heat, add remaining oil, diced onion, carrot, and celery. Sauté, stirring often until onions are soft and clear and carrots and celery are just turning slightly golden, about 15 minutes. Add a splash of water, if necessary, to keep vegetables from sticking. Add sun-dried tomatoes and harissa and stir in. Then add vegetable stock and bring to a gentle boil. Remove from heat and set aside while vegetables in oven finish roasting.