Slice lemons 1/8-inch thick. Layer half the lemons slices and shallots in a pan. Sprinkle with 1/4 cup of salt. Repeat layering process with remaining ingredients. Cover the pan with plastic wrap making sure the plastic touches the top layer of lemons. Leave the pan overnight or up to 48 hours. Rinse the lemon slices and use as you would preserved lemons. Keep refrigerated up to a week.