Preheat oven to 160 C (320 F). Line a baking tray with aluminium foil and tip the vegetables, onion, garlic, and thyme leaves onto it. Mix together the harissa paste, oil, salt, pepper, coriander seeds and cumin seeds in a small bowl. Pour over the vegetables and toss to coat all the pieces evenly. Roast for 55-60 minutes and set aside to cool slightly.
Once cool enough to handle, blend the vegetables with stock or water in a blender in batches. Check seasoning and adjust. Reheat gently with more stock or water to get the right consistency before serving.
*Some harissa brands might be more potent than others, so use accordingly.
-If the soup gets a little too spicy, stir in a dash of milk or coconut milk in the end to temper the heat.