To make the simple syrup, combine the sugar, water, grapefruit peel and sage leaves in a saucepan. Bring to a simmer over medium heat and simmer until all the sugar disolves and the mixture is thick and syrupy. Put through a strainer and cool this completely. You can store this in a jar in your fridge for weeks.

To make the cocktail, in a cocktail shaker combine the simple syrup and gin. Shake well. Pour into two glasses over ice and top with soda water and mix gently.

For the garnish, lay the grapefruit rounds on a heatproof surface and sprinkle sugar over the top. The whole thing should be completely covered in sugar - it will end up being more sugar than you think. Torch the sugar until bubbly and caramel in color. Let cool until the brunt sugar is hardened and shiny (kind of will look like glass, think top of the french dessert creme brulee).