We all know we're supposed to eat as many different fruit and vegetables as possible, because they offer an array of health benefits, boosting our immune system.

Celeriac's just one great example of a vegetable that probably doesn't figure on very many people's shopping lists.

Try it mashed with potatoes and garlic. Or when you've got a bit more time on your hands, in our great warming winter dish, celeriac and potato pie.

WHAT DO I NEED

Serves 4-6 200g (6 1/2oz) plain flour 1/2tsp salt 125g (4oz) cold butter, cut into cubes 2-3tbsp ice water 1 large or 2 medium celeriacs, peeled and cut into 12mm (1/2in) dice 450g (1lb) potatoes, peeled and diced 2tbsp butter 60g (2oz) chopped yellow onion 4 stalks celery, chopped 2tsp plain flour 750ml (24fl oz) low-sodium chicken stock 2tbsp finely chopped fresh flat-leaf parsley 1tsp black truffle oilWHAT DO I DO?For pastry: Combine flour and salt in a food processor. Add butter and pulse until mixture resembles small peas. With machine running, gradually add ice water until dough comes together.

Gather dough into a ball.

Wrap in clingfilm and flatten into a disc. Place in a freezer for 30 minutes, or - if you have the time - refrigerate overnight. Combine celeriacs and potatoes in a saucepan. Add water to cover and 1/2 a teaspoon of salt.

Bring to a boil, reduce heat to medium, and cook until vegetables are fork tender, 7-10 minutes. Drain. Melt butter in a frying pan over medium-high heat. Add onion and celery and saute until onion is translucent, 4-5 minutes. Sprinkle with flour and 1/2 a teaspoon of salt and stir well.

Gradually stir in stock and cook until reduced to about 450ml (16fl oz). Stir in parsley. Remove from heat and set aside. Preheat oven to 200C (400F). On a lightly floured surface, roll out pastry disc into a round about 3mm (1/8 inch) thick and trim to fit a 1.5 litre to 2 litre (2 1/2 to 3 1/2pt) pie dish. Put potatoes and celeriac in casserole or pie dish.

Pour reduced liquid over them. Drizzle with truffle oil. Cover with pastry, pinching edges to seal. Cut two or three slits in top. Bake until golden.

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