In a large pitcher, combine the elderflower liqueur, apple cider, rosemary syrup, and fresh rosemary leaves. Stir to combine.
Add the apple slices and cinnamon stick. Refrigerate until ready to serve.
Just before serving, slowly pour in the chilled Prosecco and gently stir.
Fill individual glasses with ice and pour in the spritz.
Top each glass with a splash of soda water and garnish with a fresh rosemary sprig and apple slice.