3/4 oz St. Germain Elderflower Liqueur
3/4 oz Aperol
3/4 oz Lemon Juice
Shake with ice and strain into a cocktail glass. The drink is often garnished with a long lemon twist. Photo below is not of the drink that I had; it is a photo Josh Childs sent me.
Shaddock: a large yellow grapefruit-like citrus fruit allied to the orange and the lemon and presumed to be native to Malaysia and Polynesia. Named after Captain Shaddock, the 17th century English ship commander who introduced the citrus varietal to the West Indies.