In a small pot over high heat, combine water, hibiscus flowers, and ginger. Bring to a boil and simmer for 20 minutes. At the last minute of simmering, add the sugar and stir until melted. Strain the syrup to a heatproof bowl and let cool to room temperature. Reserve half of the flowers in a small bowl and let cool to room temperature.
Once the syrup and flowers have cooled, cover with plastic wraps and keep in the fridge.
When ready to serve, combine the syrup, ginger ale and Prosecco in a large pitcher or glass bowl. Add ice, reserved flower petals and lemon slices if using. Serve immediately.