Three measures of Gordon's, one of vodka, half a measure of Kina Lillet. Shake it very well until it's ice-cold, then add a large thin slice of lemon peel.
The original recipe called for Kina Lillet. Lillet Blanc (also known as Lillet Blonde)
Dry Vermouth (e.g. Cinzano Extra Dry) may be used in place of Lillet Blanc.