2 large lemons
1 cinnamon stick
The Night Before
Scrub the fruits and trim the outer peel from lemons and limes with a vegetable peeler. Cut the peel into thin strips. Cut off the inner white peel from all fruits, reserving the lemon peels in a cheesecloth bag and discarding the lime peels. Thinly slice the fruits and combine with the zest.
Measure the fruit and peel and add it with an equal volume of water to a heavy, non-reactive 5-quart pan. Add the cinnamon stick and cheesecloth bag of lemon peels. Cover and bring to a boil. Uncover and simmer for 15 minutes. Let the mixture cool to room temperature, cover and let stand overnight at room temperature.
The Next Day
Remove the cheesecloth bag and squeeze out juice into the marmalade base before discarding bag. Measure fruit and liquid and set aside an equal volume of sugar. Bring mixture to a boil and add sugar, 1/2 cup at a time, waiting for the liquid to return to a boil each time before adding more. Cook until marmalade reaches the jell point, which is 8 degrees above the boiling temperature measured on your thermometer. This will take 5 to 10 minutes.
Pour the marmalade into a 1-quart measure and let sit for 5 minutes, stirring occasionally. Remove cinnamon stick. Stir down the fruit pieces.
Ladle mixture into hot, sterilized jars to within 1/4 inch of the rims. Wipe the rims clean, attach new lids and screw caps on tightly. Invert jars briefly to vacuum seal or process in a boiling water bath, submerged by 1 inch, for 10 minutes.