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Take 2 pounds of the freshest lemons you can find--not overripe or stale because the younger fruit has more pectin in it. Slice off the stem-end of the lemon but leave the blossom end intact as that’s the end that has more pectin in it. Using a blade on a food processor or a mandolin, slice the lemons as thinly as you can and throw them into a large stockpot with 3 quarts of water.
Bring the lemon water and zest to a boil and then reduce your heat and gently boil things for 30 minutes. Remove the pot from the heat and let the mixture set for 30 minutes (you can let it set longer if necessary--overnight if you must). Strain the lemons, seed and pulp from the liquid through three or four layers of cheese cloth. This should give you about 5 or 6 cups of liquid (the recipe said 5 but I got 6 and it worked fine for me).
Now here’s the trickiest part:
Combine the liquid, 2 cups of fresh lemon juice and 6 cups of sugar back in the stockpot and bring to a boil. Immediately once everything is well combined you’ll notice that the juice is suddenly light and clear and shiny. That’s your pectin reacting with the sugar and the acid from the lemon juice and it’s supposed to do that. Trust me, it’s very pretty.
Boil the mixture fairly hard (though not too aggressively, you don’t want it overflowing or going too foamy on you), stirring frequently, for 10 minutes or until the jelly begins to “sheet off the back of a spoon.” I had no idea what “sheeting” meant so I wasn’t quite sure when the jelly would be ready. After ruining a batch I figured it out.