Add the fresh blueberries, blueberry syrup, and lime juice to the bottom of a tall glass.
Muddle the blueberries firmly to break them down and release their juice.
Add the mint leaves and press lightly just a couple of times to release the oils without bruising them.
Fill the glass with ice.
Top with soda water and stir gently.
Garnish with a sprig of mint, a few blueberries, and a lime slice.
Notes
Blueberry Syrup:To make the blueberry syrup: combine 1½ cups blueberries, 1 cup sugar, 1 cup water, and 1½ tsp lemon juice in a saucepan. Simmer 10 minutes, strain well through a fine sieve, and cool completely. Full recipe at blueberry simple syrup.
Muddle the blueberries first to break them down, then add the mint and press lightly just a couple of times.
Fresh blueberries give the drink a different texture and flavor to just the syrup.
To make for a crowd, multiply and build in a pitcher. Add soda water just before serving.