Preheat the oven to 180°C, fan 160°C, gas 4. In a large saucepan, heat 1 tablespoon of the olive oil. Add the onion and cook for 5 minutes. Add half the harissa paste and the sweet potatoes. Cook, stirring, for 10 minutes.
Tip in the tomatoes, chickpeas, 200ml cold water and half of the coriander. Simmer for 15 minutes. Add the lemon juice and beans. Simmer for a further 5 minutes.
Meanwhile, brush the frozen fish with the remaining harissa paste. Put on a large baking sheet with the aubergine, and drizzle over the remaining olive oil. Bake for 15 minutes until the fish is cooked through.
Stir the aubergine into the tagine. Divide between bowls, top with the fish and garnish with the remaining coriander. Serve with the lemon wedges and flatbreads.