Trim the stem end off of the lemon. Quarter the lemons lengthwise and remove the seeds. slice the lemons as thinly as possible. Place the lemon slices in a non-reactive large pot.
Finely chop the seeds and stem ends (this can be done by hand or in the food processor). Place the chopped bits in cheesecloth or a coffee filter and tie up with a small bit of kitchen twine. Place the seed pouch in the pot with the chopped lemons. Add 2-3 cups of water, the water will not cover the lemons, but will make them look like a moist soup. Allow this lemon water to sit covered, overnight.
The next day, place the pot on the stove top, and bring to a boil over high heat. Turn the heat down to low, and simmer for 10-20 minutes or until the peels are soft. Turn off the heat, and set the cooked lemon mixture aside for an hour to cool and let the pectin develop.
Meanwhile, prepare (get out and make sure they are squeaky clean) the jars you'll use to store and/or give away the marmalade.
Take the bag of seeds out of the pot. Using a large measuring cup, see how much lemon mixture you have, and combine equal parts lemons and sugar (for a sweet marmalade) or ½ as much sugar to lemons (for a tart marmalade - I made a tart version). Add 1/2 of the lemon verbena and the vanilla or vanilla bean (halved lengthwise and pulp scraped out).
Let the mixture simmer again on medium high, stirring fairly often, until it reaches 220 degrees on your candy thermometer, about 50-60 minutes. Add the remaining lemon verbena in the final 5 minutes of cooking. Remove the vanilla bean pod, if used.
To test for doneness, dip a cool metal spoon into the hot fruit. Immediately lift it out and away from the steam and turn it horizontally. At the beginning of the cooking process, the liquid will drip off in light, syrupy drops. Try again a minute or two later, and the drops will be heavier. The jam is done when the drops are very thick and two run together before falling off the spoon. Another test for doneness is when the marmalade drips off a wooden spoon in one big sheet instead of single drops.
Transfer cooked marmalade to clean ready jars. I don’t actually go through the canning process, but if you would like to do so, please follow the instructions from the USDA’s National Center for Home Food Preservation or the official site from the makers of Ball jars.