Wash the leeks and shred them finely. Cut the prunes into sticks. Melt half the butter in a frying pan, when frothing add the leeks and 1-2 tablespoons of cold water. Season with salt and black pepper. Mix well, cover and cook quickly stirring occasionally until tender. Add the prunes and toss together. Stir in the remaining butter and the lime juice. Serve at once.