To make simple syrup: combine sugar and water in a small saucepan. Add blackberries. Tear sage leaves a bit to break and release some of their oils. Stir and bring to a boil over medium-high heat. Remove from heat and strain syrup into a mason jar. Press onto the sage and blackberries with the back of a spoon to release as much liquid as possible. Chill jar in the freezer for 20 minutes until cool.

To make the cocktail: add 3 blackberries to a short glass. Muddle berries gently to break up. Add a handful of crushed ice, simple syrup, tequila and lime juice. Top off with seltzer. Garnish with extra lime wedges and blackberries to serve.