Place the lentils in a stewpan with the water and the onion (cut in four), peppercorns, and mace, tied up in a small piece of muslin. Boil three-quarters of an hour, remove the flavourings, add salt, and simmer for another quarter of an hour. Strain, rinse the stewpan, pour back the sauce, and thicken with the butter and flour.
Note. The lentils should not be thrown away, but are just ready for converting into sausages, etc.