The method is simple: cut the stalk off a fresh marrow (a marrow is a squash, similar to a large zucchini) and scoop out the seeds leaving as much flesh behind as possible. Pack the resulting opening with sugar (most recipes use Demerara - or raw cane - sugar). Replace the top, puncture the bottom and suspend over a bowl or some other receptacle. Most recipes I've seen depend on natural yeasts entering the marrow and fermenting the fruit, but I suspect adding yeast may be required. Some call for the addition of chopped ginger or raisins to add flavour. Fermentation time seems to range from a week up to several months, depending on the recipe!
A sample recipe:
Whether the ensuing brew is a tasty potable, or instead something worthy of its British cooking heritage is unclear but it seems like a good candidate for the next bayblab project. Perhaps other fruit would make good candidates for recipe adaptations.
Pair ladies' nylon stockings
Slice the top off the marrow and scoop out the seeds, but leave as much of the flesh as possible, and don't puncture the skin. The seeds are attached to the inside of the marrow with little strings: one way to detach obstinate seeds is to fill the marrow with water and shake well. Fit a stocking onto the marrow, and pack the marrow tightly with Demerara sugar. Stand upright in a large vase or similar and leave for a day. Osmosis will draw fluid from the marrow into the sugar, dissolving some and leaving a gap below the end of the marrow. Add a teaspoon of ginger (finely chopped fresh ginger root if available, otherwise dried ginger) and pack more sugar on top. Replace the marrow upright in the pot, and cover to ensure nothing can fall into the pot or the end of the marrow. Pulling the second stocking down over the top of the marrow and pot is a good way to do this. Leave in a cool dark place for at least two weeks, and if possible, four. Inspect at intervals.
The idea is that the sugar ferments, generating an alcoholic liquor which gradually eats its way through the bottom of the marrow and collects in the pot. It takes three or four weeks to reach full strength, during which time the marrow becomes very floppy. The stocking filters the liquor and also provides much-needed surgical support. I have tried this recipe three times, twice with marrows and once with a Turk's Head Turban gourd, also from Elder Stubbs. One week's fermentation gives a liquor which has a pleasant taste but is not very strong - after three weeks, it has a noticeable punch. The ginger adds pungency to counteract the blandness of the marrow, giving the rum a pleasant warmth.