Preheat the oven to 180° fan/200°C/gas 6. Tip the sweet potato, ground cumin, chilli powder, sea salt and oil into a large roasting tin, then transfer to the oven to roast for 25 minutes.
Toast the panko breadcrumbs in a small, dry frying pan for 2–3 minutes until golden brown.
Transfer to a plate to cool.
Cook the peas in a saucepan of boiling salted water for 5 minutes. Drain well, then mash roughly with a potato masher. Heat the oil in the frying pan you used for the breadcrumbs and add the grated ginger. Stir-fry for 30 seconds, then add the crushed peas and stir-fry for 2–3 minutes. Stir through the ground roasted cumin seeds, yogurt and lemon juice, then taste and add salt as needed. Set aside.
Once the potatoes have had 25 minutes, remove them from the oven, and make space for the 4 fish fillets. Spread the mustard over each piece of fish and top with the toasted breadcrumbs. Drizzle over the oil, then transfer to the oven to bake for 20–25 minutes until the top is just golden brown and the fish is cooked through. Serve the fish and chipped sweet potatoes with the crushed peas on the side.
NOTE: See page 86 for more on ground roasted cumin seeds. It’s such a versatile seasoning and takes minutes to make – a million miles from ordinary ready-ground cumin.