Wash the lemons well and trim off the ends. Arrange them in a single layer in the bottom of a large pan that has a lid. Cover the fruit with water, tuck in the bundle of lavender amidst the fruit, and set the pot on the stove over high heat.
Bring it to a boil and reduce the heat to medium. Let the lemons simmer for approximately 20 minutes. They're done when they're tender but not falling apart. Let them cool completely (I often let them sit overnight), taking care to reserve the water (it will be the liquid component in our jam.
Once the lemons are cool, remove the lavender bundle and set a fine mesh sieve over a bowl. Using a paring knife, cut each lemon in half over the sieve and wiggle the seeds out.
You want to catch the seeds in the sieve and have the juice run through to the bowl. You may end up with a goodly amount of pulp in the sieve as well. Work the pulp around in the sieve to push it through.
When all the lemons are cut in half and deseeded, heap them in a blender container (if you have a small blender, work in two batches). Measure out six cups of cooking water (add some fresh, if you don't have enough) and add it to the fruit.
Blend until you have a mostly smooth lemon puree. Pour it into a low, wide pan and add the sugar.
Bring to a boil over high heat and reduce to medium-high.
Cook, stirring regularly, until the jam thickens and sheets off the back of your spoon or spatula. You can tell it's nearly completion when it hisses and spits when you stir. My batch took a little over half an hour of vigorous boiling to achieve set, but times will vary.
When jam is finished cooking, remove pot from heat.
Funnel jam into prepared jars. Wipe rims, apply lids and rings, and process in a boiling water bath canner for ten minutes.
When time is up, remove jars from canner and let them cool on a folded kitchen towel.