2 skinless white fish fillets (look for sustainably caught)
Heat oven to 200C/fan 180C/gas 6. Cook the sweet potatoes in boiling, salted water for about 10 mins or until just tender when pierced with a knife. Meanwhile, mix together the butter with the garlic, harissa, lemon zest, chopped coriander and some seasoning. When the potatoes are ready, drain thoroughly, mash with the ginger and seasoning, then keep warm.
Place the fish in a roasting tin, season, then spread half the flavoured butter over each fillet. Roast for about 8 mins until just cooked through. Serve with the ginger mash and some green veg.