Mix the gin, Cointreau, lemon juice, and simple syrup in a pitcher or shake together in a large mason jar and chill.
Taste and add more lemon or syrup to adjust if needed.
Place a couple of fat ice cubes in a rocks glass, fill ⅓-½ way with the gin mixture and top with prosecco.
Garnish with a lavender sprig or a lemon peel and serve!
