Potato, celeriac and leek gratin with sage and feta

This is here as a guide to making gratins. Most root vegetables can be cooked like this, layered in a dish and flavoured with anything from herbs and cheeses through to spices and nuts. Then, you pour over boiling water, stock or double cream, seal tightly with foil and bake until the vegetables are cooked. The top may then be coloured under a grill. Choose a dish just large enough to hold everything, but one in which the liquids won’t boil up and out of the dish. Cut out a sheet of non-sti