The classic 1:1:1 (equal parts gin, vermouth, and Campari), but Chiltern Firehouse in London tweaked the ratio on this bittersweet Italian classic.
Stir gin, vermouth, and Campari in an ice-filled mixing glass until very cold, about 30 seconds. Strain cocktail through a Hawthorne strainer or a slotted spoon into an ice-filled rocks glass.
Using a small serrated knife, remove a 1" strip of peel from the orange (some white pith is okay); it should be stiff enough to provide some resistance when bent. Twist over drink to express oils; discard. Garnish with 3 very thin orange slices.
1½ ounces gin
1 ounce Carpano Antica Formula vermouth
¾ ounce Campari