Wash Lemons: Remove any wax or pesticide residue from the lemons by scrubbing them with a little dish soap and rinsing thoroughly under warm water.
Slice: Cut off the ends of the lemon, then cut the lemon crosswise in 1 cm slices. Do not slice them thinly, or they may fall apart during the cooking process.
Blanch: Fill a Dutch oven with two inches of water, then bring to a rolling boil. Remove it from the heat, add the lemon slices, and stir gently. Allow the slices to sit in the hot water for 1 minute, then immediately drain with a skimmer and place in a bowl of ice water to halt the cooking.
Simple Syrup: Discard the blanching liquid and rinse out the pot. Pour the filtered water and granulated sugar into the pot and stir over medium heat until the sugar is dissolved. Bring the syrup to a simmer.
Simmer: Add the lemon slices, making sure they are not overlapping, and cover and simmer gently on low heat for 1 hour. Be sure not to boil, or the fragile flesh of the lemons may disintegrate. The peels should be soft and translucent.
Dry: Spread the candied lemon slices on a wire rack and set over a cookie sheet to catch any drips of syrup. Air dry at room temperature for 1-3 days, or bake them at a low temperature (175-180°F) for 2 hours, or until tacky to the touch.
Serve: Roll the candied lemon slices in extra granulated sugar to reduce tackiness and for a pretty frosted look. Cool completely on the wire racks, then store in an airtight container until ready to use. Serve as a garnish on cakes, cupcakes, or sweet loaf breads, or enjoy as a candy.