Add the Raspberry Syrup and lemon juice to the bottom of a champagne flute. Fill the glass with prosecco.
Place the sugar and 2 cups water in a medium saucepan over medium heat. Bring to a simmer, stirring occasionally, until the sugar is dissolved, 3 to 4 minutes. Add the lemon juice and raspberries and simmer, stirring occasionally, until the raspberries have broken down, about 10 minutes. Pour the mixture through a strainer and return to the pot. Simmer over medium heat until the syrup has thickened, about 10 minutes more. Let cool completely, then pour into an airtight container and store in the refrigerator for up 2 weeks.