Muddle lemons, mint and syrup in a shaker. Add whiskey and Shake. Double strain into a rocks glass over fresh ice. Garnish: mint sprig.
In PDT book, it calls for Rittenhouse, but Meehan credits Degroff. In "Craft of Cocktail" Degroff's older version is made the same but: 2 lemon, 2-3 mint, .75 syrup, 1.5 Maker's Mark bourbon, 1oz water.
Apparently based an old recipe by Jerry Thomas, modified with the inclusion of citrus by Dale Degroff. See comments for more.
Meehan, Jim. PDT Cocktail Book. 2011. p. 265