Add the hulled strawberries and cherry blossom syrup to a cocktail shaker.
Muddle until the strawberries break down and release their juice. If using basil, add the leaves now and muddle gently alongside the fruit.
Add the gin, lemon juice, and ice.
Shake hard for 15 to 20 seconds until fully chilled.
Double-strain through a fine-mesh strainer into a rocks glass filled with fresh ice. Garnish with a strawberry slice on the rim.