Fill a shaker or pint jar with ice. Pour in lime juice, gin, mint leaves and two candied grapefruit peels. Cap and shake vigorously for 1 minute. Slightly wet half the rim of highball glass then dip into fleur de sel. Pour cocktail into glass, top with soda, then garnish with candied peel and lime slice.
Candied Grapefruit Peel
Place the grapefruit peel in a small saucepan and cover with water by an inch. Bring to a boil and then drain. Repeat three more times. This will take away a lot of the bitterness in the peel.
Clean out the same saucepan and add the sugar, bay leaf, thyme, ginger and two cups of cold water. Bring to a boil and then lower to a bare simmer. Add the grapefruit peel and slowly cook for an hour and a half. Remove from heat and transfer the peel in the syrup to a clean jar.