Slice the lemons 1/4 inch thick and set aside.
Add the lemon juice, sea salt and 1/4 cup water to a saucepan and bring to a boil to dissolve the salt. Add the lemon slices, chiles and 1 of the rosemary sprigs and boil for 1 minute. Turn the heat to low and simmer until the lemon rind is translucent, about 10 minutes. Let cool.
Transfer the lemon slices to an airtight jar. Remove the chiles and rosemary from the pot and discard. Pour the liquid from the pot over the lemon slices just to cover. Add the remaining rosemary sprig. Cover tightly and keep refrigerated for up to 2 weeks.