Add the gin, syrup, juices and bitters into a shaker with ice and shake until chilled.
Strain into a chilled flute.
Top with the chilled prosecco.
*Pineapple syrup: Peel and core 1 whole pineapple, then cut the meat into chunks. Add 1 cup white sugar, toss to coat in a glass bowl and store, covered, in the refrigerator overnight. Combine 1 cup sugar and 1 cup water in a saucepan, simmer until the sugar is dissolved, and remove from the heat. Blend the macerated pineapple in a blender, add the simple syrup, and stir to combine. Strain out solids using a cheesecloth, and store the syrup in the refrigerator.