Combine the gin, soda and lavender tincture in a mixing glass
Mix the ingredients together and pour into a cocktail glass
Garnish with one scoop of tonic sorbet (recipe below)
To Make the Sorbet
In a small saucepan, over medium heat, combine the water and sugar, and boil until the sugar dissolves
This should take about 1 minute
Remove from the heat and allow to cool
Stir in the lemon juice and lemon zest, then pour into the bowl of an ice cream maker
Churn according to manufacturer instructions
When the sorbet has frozen into ice crystals (it will still seem quite soft), transfer to a storage container, cover tightly and freeze until ready to serve