Wash the vegetables, drain well and pat dry. Prepare a bowl of cold water with some lemon juice.
Trim the artichokes of their tough outer leaves. Chop off about a third of the top spear. Cut off the stem, leaving about 3 cm (1¼ in) and trim down towards the end into a pyramid point, trimming away the dark outer green stem. Divide lengthways into quarters. Trim away the choke. Put them i