In the bottom of a cocktail glass, lightly muddle together basil leaves and lemon juice.
Add gin and stir.
Fill the glass with ice.
Top with tonic water.
Give it one final stir before serving with a lemon twist.
notes
Be sure to only lightly muddle the basil leaves as they can turn bitter very quickly. You don't want to muddle them so vigorously that they break apart - just enough to release the flavor.