Under the skin
Celeriac isn't, as a friend thought, gluten intolerance (that would be coeliac disease). Also known as celery root, celeriac doesn't look appealing - it's bulbous, heavy and dense - but the flavour and scent are quite delicate, reminiscent (as you'd expect from the name) of celery. Celeriac is a relative of the plant that's raised for celery stalks.
To prepare celeriac, use a sharp knife to cut away the bumpy skin. Some shops sell the root prepared, revealing the hard, pale ivory-coloured flesh.
When paired with potatoes, celeriac makes a delicious, subtly flavoured, lightly textured mash. Peel the potatoes and celeriac and cook them separately in simmering salted water until tender. Drain thoroughly then mash the vegetables together with butter, cream, salt, pepper and a little freshly grated nutmeg.
Celeriac and potato gratin has similar ingredients. Thinly slice the peeled root vegetables and blanch separately for about two minutes in simmering, salted water. Drain thoroughly. Layer the celeriac and potatoes in a well-buttered gratin dish, season with salt, pepper and grated nutmeg and pour cream over the top, just over halfway up the sides of the dish. Sprinkle with grated gruyere and bake until the vegetables are tender and most of the cream is absorbed.
For a delicious cold salad to serve with roasted meats, peel celeriac then julienne or grate it into long, thin strands. Sprinkle liberally with salt and fresh lemon juice and mix to combine. Leave for about 15 minutes then rinse the celeriac with cold water, drain and squeeze out the excess liquid. Mix prepared mayonnaise with Dijon mustard, white wine vinegar and salt and pepper to taste, then dilute the mixture to a light coating consistency. Pour over the celeriac and mix before serving.