Preheat the oven to 400F/200C.
Peel the sweet potato and carrot and cut each into roughly bite-sized chunks (make the carrot a little smaller, as they are slower to cook). Peel the onion and cut into wedges. Remove seeds and stem from the pepper and dice (roughly 1in/2.5cm pieces).
Chop the stem end off the eggplant/aubergine and cut into wedges - you want the pieces bigger than the root vegetables as they will shrink down more but still not huge. Place all of the vegetables on a baking sheet/tray.
Mix together the harissa and 1 tablespoon of the olive oil and stir to form a smooth paste. Drizzle it over the vegetables then toss them so they are well coated with the spiced oil. Spread the vegetables out so they lie in a single layer - this will help them roast rather than steam as they cook. Drizzle the remaining oil over the vegetables, especially on the eggplant as they can dry more as they cook, or any pieces that look less coated with oil.
Place the baking sheet in the oven and roast for around 40 minutes in total until the vegetables are tender starting to brown slightly. Turn the vegetables roughly halfway through cooking.
As the vegetables are nearing being done, prepare the sides, if adding. Put the couscous in a heatproof bowl, add a small drizzle of olive oil and pour boiling water over it to just cover the level of the couscous by a small amount. Cover the bowl and leave for a couple minutes to absorb the water. Then fluff up with a fork.
For the yogurt sauce, finely chop the cilantro/coriander and parsley and add them to the yogurt, along with the lemon juice, oil and a little salt, pepper and garlic powder, and mix well.
Serve the vegetables over the couscous with the herby yogurt sauce to the side or drizzled over.