Yuzu is a citrus fruit found in East Asia. One of its many uses is in yuzucha, which resembles marmalade and is most often mixed with hot water to make yuzu "tea," which of course is not a tea at all but is delicious and warming anyway!
To make a big jar of the stuff:
Wash yuzus, cut into quarters, remove seeds, peel. Tear remaining innards into small-ish pieces and put into a glass container big enough to hold all the ingredients. Thinly slice peels and add to container. Add sugar, mix well, and cover.
The yuzucha is "done" when the sugar is dissolved. Since I used coarse beet sugar, this took only a couple of hours (with occasional mixing). If you use rock sugar, it will take longer. In either case, this probably tastes best after a week.
Keeps about 3 months in the fridge.
For a warm, energizing winter drink, mix a spoonful into hot water. Other uses are also possible.