Fill a saucepan two-thirds full with water and bring to a simmer.
Bring the simple syrup to a gentle simmer.
Slice the lemons very thinly with an electric slicer. Blanch in the hot water until translucent, about 10 minutes. Remove from the pot with a spider and blanch once more in fresh water for ten minutes.
Place the slices in a single layer in a hotel pan. Pour the simple syrup over the lemon slices so that they are generously coated. Let them sit in the syrup for at least 1 hour and up to 1 week.
Remove each slice from the syrup, letting any excess drip off of it, and then place them all in a dehydrator at 57°C / 135°F until dry and crisp. They can also be dried in a convection oven on a nonstick rubber mat at the same temperature with the vent open.
Carefully lift the slices from the nonstick rubber mat with a rubber spatula and let them cool and harden.
Store at room temperature in an airtight container. Use silica gel packs to absorb any moisture from their environment in order to prevent them from softening. In dry conditions, they can last up to 1 month.