Soak the whole esrog for ten days. If you are using multiple esrogim, they may be soaked altogether in the same bowl. Change the water daily.
Cut the esrog into slices crosswise, approximately 1/3” thick. Remove the numerous seeds and discard. If you’ve never sliced open an esrog, you’ll be exceedingly surprised at the multitude of seeds and the thickness of the skin. It seems to have almost no flesh at all. Place the slices in a large bowl and cover with water. Let it soak once again overnight in a cool place.
The next day, drain the water. Place all the slices in a large pot and cover with fresh water. Bring to a boil and pour off the water. Repeat this “boil and drain” procedure nine more times for a total of ten times. This will help tone down the bitterness of the fruit.
Next, the esrog is cooked in a sugar solution. For each esrog use the following proportions.
Bring the sugar and water to a boil. Pour over the drained esrog slices. Let this mixture stand at room temperature a few hours or overnight.
The next day, cook the esrog and sugar mixture over a medium-low flame for 2½ hours, checking it frequently to be sure that the sugar is not darkening. Lower the heat if necessary. Add the lemon juice and cook a half hour longer.
While the mixture is cooking, prepare your jars for canning. Use two half pint jars per esrog. Place the clean, empty jars in a large stockpot and fill with water. Heat the water and boil for 10 minutes. In a separate, smaller pot, heat jar-lids and sealing rings just until hot. Do not boil the lids as it will damage the seal. Using canning tongs or locking tongs, lift the jars out of the hot water, one at a time. Fill each jar with the hot esrog mixture to ¼ inch below the top. Close jar with hot sterile lid. Return the jars to your stockpot and bring to a boil once again. Cook for 5 minutes and remove from the water. Wait for the distinctive “pop” that lets you know the jar is sealed. Cooling the jars upside-down helps facilitate sealing. Fastening the jars in this manner allows you to store them without refrigeration for a long period of time. If you prefer, just screw on the lids and refrigerate until needed. The esrog jam will last 3-4 months in the refrigerator.