Thoroughly wash the lemons, then use a mandoline or sharp knife to slice to 4-5mm thickness*, discarding the ends. Remove any seeds.
Bring a large sauté pan or pot with 6 cups (1.36 kg) of water to a boil. Boil the lemon slices for 2 minutes, then transfer the slices to an ice bath and discard the boiled water.
In the same sauté pan or pot, bring 6 cups (1.36 kg) of water and 3 cups (594 g) of granulated sugar to a boil. Add the lemon slices, then reduce heat to low and simmer for 20-25 minutes, stirring occasionally to ensure the slices are evenly coated in the liquid. The rinds should be soft and look slightly translucent.
Using a slotted spoon or spatula, carefully remove the lemon slices from the liquid and spread evenly on a drying rack (see notes below if using a dehydrator or drying in the oven). Allow the slices to dry at room temperature for 12-24 hours, flipping occasionally. The lemons will feel sticky, but not wet, when they're ready for the sugar coating.
If you'd like to keep the liquid (it's now Lemon Simple Syrup, which is great for cocktails), let it cool fully, then strain into a jar or airtight container. Store in the fridge for up to 3 weeks.
After the lemon slices have dried, gently toss them in ½ cup (99 g) granulated sugar to coat. Serve immediately or store in an airtight container in the fridge for up to one month.
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Notes
*If your lemons feel too soft to slice evenly, place the whole lemons in the freezer for about 30 minutes. This will help them keep their shape and be a bit more firm while slicing. Drying with a dehydrator: If using a dehydrator, transfer the lemon slices to the dehydrator after simmering. Set to 135 degrees F and dry for 3-4 hours. Remember the slices should be slightly tacky and not completely hardened or dried so the granulated sugar will stick. Drying times will vary based on the thickness of your slices. Drying in the oven: If you want to dry the slices in the oven, transfer the simmered slices to a parchment paper-lined baking sheet. Set the oven to 200 degrees F and dry for 1-3 hours, flipping every 30 minutes to check the dryness. Remember the slices should be slightly tacky and not completely hardened or dried so the granulated sugar will stick. Drying times will vary depending on the thickness of your slices.