Add the butter and caster sugar and stir. Then transfer to a 900ml/1½ pint capacity ceramic gratin dish, about 5cm/2in deep. Leave to cool while you prepare the topping. For the topping ...

A gratin is comparable to a baked mac ... you'll want to add your vegetable base layer to a greased ceramic or glass (not metal) baking dish. You may need to partially cook your vegetables first ...