Shake with ice, strain into a rocks glass, fill with fresh ice, float 1/2 oz Genever, and add a straw.
Our ginger syrup at work is equal volumes chopped ginger (skin on) and 1:1 demerara syrup that is blended and strained. It does not come across as sweet and requires extra sugar content to balance citrus. Adjust the citrus level accordingly to balance your ginger syrup.
A guest at Drink wanted a gin cocktail but described a Penicillin. Therefore, I made it as such with a London Dry in the mix and a Genever as the float (albeit more hefty than the smoky Scotch amount in the original). They enjoyed it so much that I made one that night when I got home. For a name, I searched around for a Dutch-themed antibiotic with little to show for my efforts, so I dubbed this one the Erasmus after a famous Dutch philosopher.