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.