In a cocktail shaker, add the fresh hulled strawberries and the simple syrup. Muddle gently.
Add the gin, fresh lemon juice, and the egg white to the shaker and shake well (this is the dry shake).
Add a few ice cubes to the shaker and shake for another 15 seconds (this is the wet shake).
Strain into a highball glass filled with ice and top very gently with the soda water. Garnish with a strawberry slice on the rim and serve immediately.