Greenshell, or green-lipped, mussels come from New Zealand and they are larger and more meaty than the smaller European black mussels. If you can’t find them fresh, frozen ones in the half-shell work very well – although, as they are already cooked, they’ll just need brief reheating. Failing that, use the European mussels. Use any small potato you like, although personal favourites for this salad are Anya, Pink Fir and Baby Pearl.