Add the gin, fresh lemon juice, simple syrup, and pasteurized egg white to a cocktail shaker. Do not add ice or soda water yet.
Dry shake vigorously without ice for 15-20 seconds to aerate the egg white and create a thick foam.
Add ice to the shaker and shake again for 10-15 seconds, or until the cocktail is well chilled.
Fine-strain the mixture into a chilled Collins or highball glass.
Slowly top with 1-2 oz chilled soda water, pouring gently to help lift and preserve the egg-white foam.
Garnish with a lemon peel if desired and serve immediately.