Combine the strawberries, lemon juice, and granulated sugar in a blender and mix on high power for 2 minutes, or until smooth.
Transfer to an airtight container and refrigerate overnight.
When ready to serve, pour the strawberry puree into a pitcher. Pour the Prosecco over the top and stir. Serve immediately.
Notes
You can also pour the strawberry puree through a fine mesh sieve before chilling to remove any remaining solids/seeds for a smoother drink. But I'm kind of on the lazy side of things.