1 1/2 oz Tanqueray Ten, 1/2 oz. Grand Marnier, 1/2 oz agave nectar, 1 oz. graperuit juice, 1/4 oz. lime juice, 1 oz. bottled tonic water. Combine all ingredients except tonic in a tall mixing glass. Add ice and shake vigorously then strain over ice into a collins glass. Top with tonic water and swirl. Garnish with a slice of grapefruit.