Wash the leeks, trim 1 cm (½ inch) off the root end and peel off the tough outer layers. Cut the white part of each leek into three 3 cm (1¼ inch) lengths. You need 20 in total. Reserve the green tops for a stock. Wash the leek lengths in cold water, dry on paper towel and place in a small vacuum pouch with the oil and a pinch of salt. Vacuum-pack on 100 per cent for 40 seconds in a chamber vacuum sealer*. Place in a saucepan of cold water and slowly bring to the boil, then reduce the heat and simmer for 8 minutes. Bringing the leeks slowly to the boil results in a more tender texture.
When ready to serve, remove the leeks from the bags, check the seasoning and add the lemon juice. Serve hot.