Combine sugar and water in a small saucepan. Heat over medium, stirring just until the sugar dissolves. Remove from heat and cool completely.
Add raspberries, cooled syrup, lemon juice, and salt to a blender. Blend until completely smooth.
Strain through a fine-mesh sieve to remove seeds, pressing gently to extract as much liquid as possible.
Chill the mixture until very cold, at least 1 hour.
Churn in an ice cream maker according to manufacturer’s instructions until thick and scoopable. (See notes for how to make without an ice cream maker.)
Transfer to a freezer-safe container and freeze 2-4 hours to firm up.
Scoop sorbet into a champagne coup or other vessel. Top with Prosecco or sparkling beverage of choice and serve immediately.