Pressed leek terrine with dijon mayonnaise

• 1.2 kg (2 lb 11 oz) young, slender leeks
• ½ small

Line a standard terrine mould or loaf tin with plastic wrap. Trim the leeks to roughly the same length as the terrine mould, leaving a little green at the ends. Rinse them well under cold running water to wash away any grit. Drop the leeks into a large pan of salted, boiling water and simmer for 10–12 minutes until completely tender to the point of a knife. Drain and leave to cool slightly.