Combine the gin, lemon juice, and simple syrup in a cocktail shaker with two cubes of ice. Shake hard until a frost forms on the outside of the shaker. Pour into a tall glass (with crushed ice if you prefer), and top with the Prosecco. Garnish with a twist of lemon rind.
French 77 Variation: Add ½ ounce of St. Germain elderflower liquor to the shaker with the gin, lemon juice, and simple syrup. Top with the Prosecco and serve!
How to Make Simple Syrup: I wrote more about simple syrups a few posts back (see the post here), but making your own syrup is incredibly easy. Combine one part sugar with one part water in a small saucepan over medium low heat. Simmer until sugar dissolves and the mixture is reduced by one-third to one-half. Let cool before using, and then store in the fridge for 3-4 weeks. I usually start with 2 cups white sugar and 2 cups water, and keep the extra in a swing-top bottle in the fridge.