Preheat the oven to 240℃.
Wash and cut the potatoes in 2 or 3 pieces. No need to peel them. Mix together with the 3 tbsp harissa and 1 tbsp of olive oil, and dump onto a lined baking tray. Cover with aluminium foil and put in the oven for 15 minutes.
While the potatoes are baking, slice the bell pepper and carrot in small pieces. Cut the onion in halve rings. Add the veggies, together with the meatballs, to the same bowl you had the potatoes in, and add in the remaining harissa and olive oil. Mix.
Take the potatoes out, remove the aluminium foil and turn the oven down to 200℃. Dump the veggies and meatballs on the tray, and put it back in the oven for another 25 minutes.
While those roast, make the tahini dressing by mixing all the ingredients together. Add some water if you need to thin the dressing a bit.
Serve the traybake drizzled with tahini dressing and some fresh herbs.