Make the batter: beat the egg yolk in a large bowl with 1/2 tsp salt and 7 fl oz (200 ml) iced water. Sift in the flour and stir just enough to get rid of any lumps. Cover and chill in the refrigerator.

Bring a large saucepan of salted water to the boil; add half the lemon juice. While it is heating, peel the scorzonera and cut into pieces approx. 1 1/2 in (4 cm) long. As each piece is prepared, drop it into a large bowl of cold water acidulated with the remaining lemon juice to prevent discoloration.

When the water boils, drain the scorzonera pieces and boil for 18 minutes or until tender but still firm. Drain well and leave to cool a little. Cut each piece in half lengthwise and remove the tough, woody central section.

Heat plenty of oil in a deep-fryer to 350°F (180°C) and deep-fry a few pieces of the scorzonera at a time, until they are crisp and golden; make sure the temperature of the oil remains constant.

Remove each batch from the oil with a slotted spoon and finish draining on kitchen paper; keep hot while you finish the deep-frying. Serve immediately.