Preheat the oven to 180°C/gas mark 4.
Peel the sweet potato. Chop the aubergine and the sweet potato into bitesize chunks. Mix with 1 tbsp of the olive oil, spread on a baking sheet and roast for about 20 minutes, turning halfway through. The sweet potato should be quite tender.
In the meantime, heat 1 tbsp of the olive oil in a saucepan. Add the chopped onion and garlic and fry for about 5 minutes to soften without colouring.
Add the chopped red pepper and mushrooms and cook for another 5 minutes.
Add the harissa and ras-al-hanout and cook stirring for a minute or two before adding the tinned tomatoes. Bring to the boil.
By this time the aubergine and sweet potato should be more or less done. Add this to the pan, mix well and transfer it to a casserole dish.
Drain the chickpeas, put them in bowl and mash them roughly with a fork. You just want to squash some of them leaving about half of them whole.
Add 1 tbsp olive oil, 1 tsp cumin and crumble the feta in. Mix well and spread out on top of the casserole.
Bake in the oven for about 25 minutes until the top has started to brown and the edges crisp up a bit.