Thoroughly rinse the cockles, discarding any shells that are cracked or broken. Scrub them well to remove any sand or grit. If the shells are open, give them a light tap; if they don’t close, discard them.
Trim away the tough green part of the leek, trim back the root at its base, then slice the leek into 0.5-1cm rounds. Carefully wash the sliced leek in plenty of cold water, drain well and set aside.
Heat a large, heavy-based pan, then add the olive oil and butter. When it’s foaming add the leek rounds. Fry gently for 4-5 minutes, shaking the pan and turning them occasionally. Add the garlic, crushed fennel and a little salt and pepper, cook for another minute, then turn up the heat to high and add the cockles, the diluted Pernod and the cream.
Put the lid on the pan and bring it up to the boil. Cook for 3-4 minutes or until the cockles are all open. A shuffle and a shake of the pan will encourage them to do so (discard any that remain closed).
Use a large slotted spoon to lift the cockles out of the pan and straight into 2 large warm serving bowls. Add the chopped dill to the simmering sauce in the pan and stir well, then taste and adjust the seasoning if required.
Spoon the leeks, along with the lovely creamy dill and fennel seed sauce, over the cockles in the bowls and serve immediately with good bread and butter and wedges of lemon for squeezing over.