In a blender, combine the fresh strawberries, basil leaves, simple syrup, and freshly squeezed lime juice.
Blend the mixture until it's smooth and well combined.
Strain the strawberry-basil mixture through a fine-mesh sieve to remove any pulp. This step is optional, depending on your preference for texture.
Fill a highball or Collins glass with ice cubes.
Pour the strawberry-basil mixture over the ice.
Top off the glass with chilled club soda or sparkling water, typically about 4-6 ounces, depending on your preference for strength and fizziness.
Stir gently to combine the ingredients.
Garnish your Strawberry Basil Mocktail with strawberry slices and basil leaves, if desired.
Serve!