Sirop de citron is a lemon syrup made by combining the juice of a lemon with the zest of said lemon. Using a single lemon will yield enough syrup for a few cocktails, but the recipe can be multiplied easily. I generally make a batch using 6-12 lemons, depending on the need. —Hi, I'm Brian.
Combine zest and juice in a bowl and refrigerate overnight.
Strain lemon juice mixture.
Combine lemon juice with an equal amount of sugar until sugar is dissolved.
Note: If you Google recipes for sirop de citron you'll find many iterations, most of them involving heating the lemon and sugar together until the sugar is dissolved. My philosophy with regards to syrups is that heat shouldn't be used unless absolutely necessary, especially if time is a more than adequate replacement to extract the flavor. Since lemon juice plus time can "cook" most anything, let it do its thing. The "slow cook" method gives the syrup a hint of vanilla that is completely surprising and delicious.
Combine gin, sirop de citron, ginger liqueur, and bitters in a cocktail shaker with ice.
Strain into an old fashioned glass and finish with soda.