Combine all ingredients except for seltzer water in a cocktail shaker filled with ice. Shake and strain into a collins glass. Add ice cubes to the glass and top up with seltzer water (approx. 4-6 fl. oz)
You've probably had one of these before at a bar, but depending on where you were it might have been made with cheap gin and sour mix and who wants that. I like the classic recipe that uses Old Tom Gin, an old school type of gin that is normally sweeter than regular gin and has spent some time aging in barrels.
If you can snag a bottle at your local store the Ransom Old Tom Gin is very nice. In a pinch, this recipe will still work well with a normal London Dry style gin.