A customary way to make use of an etrog (or esrog, or citron) after Sukkot (and to render it edible) is to cook it up into a citron delicacy like this one. Some have the tradition to serve their etrog dishes on Tu-Bishvat. There is also an old tradition that pregnant women who sample an etrog have easier births.
Ingredients:
1 etrog
2 quinces
1 lemon
6-8 cloves
2 cups sugar
Directions:
1) Soak etrog in water for seven days, changing water daily.
2) Peel and quarter etrog and remove membranes and seeds. Cube the quarters. Place into a four-quart saucepan and add water to cover. Bring to a boil and cook for five minutes, then drain all water. Repeat boiling process three times.
3) Peel and cube quinces. Wash lemon and slice thinly. Add all ingredients to etrog and add water to barely cover.
4) Cook for three to four hours over medium-low heat until reddish in color.
Note:
Freezes very well.