One 1-3/4 -oz. package powdered pectin (I used 2oz packages)
6-1/2 cups granulated sugar
Zest the lemons, avoiding as much of the white pith as possible. You’ll need 1 cup of zest strips. Put the zest in a 4-quart (or larger) saucepan.
Trim the ends from the zested lemons to expose the flesh. With one cut side down on the cutting board, trim the pith off the lemon all the way around and discard the pith. Supreme the lemons and remove the seeds.
Add the sliced lemons, ginger, and 2 cups water to the lemon zest. Bring to a boil over medium-high heat, adjust the heat to maintain a simmer, and cook until the zest is soft and the membranes start to break down, 6 to 8 minutes.
Whisk the pectin into the mixture. Increase the heat to high, add the sugar, and bring to a boil, whisking constantly to smooth lumps. Boil vigorously for 1 minute, whisking constantly (move the pan off the burner momentarily if it threatens to boil over). Remove the pan from the heat and let sit undisturbed for 5 minutes.
Skim any foam and seeds off the surface of the marmalade. Stir gently to redistribute the solids. Transfer the marmalade to heatproof storage containers, let cool to room temperature, and then refrigerate for up to 1 month.
For longer storage at room temperature, can the marmalade.