Peel the marrows.
Cut into halves lengthwise and scrape out the seeds and pith with a tablespoon.
Cut into pieces, about 2 in square.
Fat steam the pieces for 10 min., i.e. shake them in the melted fat, well below frying temperature, with the lid on the pan until the fat is absorbed.
Add the liquid and the salt, and simmer gently until the marrow is tender, about 15 min Drain well, retaining the cooking liquor for use in making the white sauce.
Dish the marrow in a hot dish and coat with the sauce.