Make the simple syrup: Combine the sugar with 1/4 cup water in a small saucepan. Bring to a boil, stirring to dissolve. Remove from the heat and let cool. (Makes 4 ounces; refrigerate up to 1 month.)
Combine the whiskey, lemon juice and 1/2 ounce simple syrup in a cocktail shaker; fill with ice. Shake well; strain into a rocks glass filled with ice. Top with the lager and garnish with lemon twist.