Heat the oven to 200C/fan 180C/gas 6. Cut the sweet potatoes into large wedges. Blanch in boiling water for 5 minutes and drain. Put the potatoes on a baking sheet with a drizzle of olive oil, half the fennel seeds and some seasoning. Roast for 15 minutes until just tender. Season the fish and sprinkle over the rest of the fennel seeds and 1 tsp oil. Add to the tray and cook for another 5 minutes. Serve with green salad.