In a medium heatproof bowl combine raspberries, sugar, and lemon juice; cover tightly with plastic wrap. With a small paring knife, poke three holes in the top of the plastic wrap.
Bring a medium saucepan filled halfway with water to a boil; reduce to a simmer. Place the bowl with the raspberry mixture on top of saucepan and let cook without stirring for 1 hour. Remove bowl from heat, and strain through a fine-mesh sieve (do not push on the berries); let cool. Divide the syrup among Champagne flutes and top each with 4 ounces Prosecco. Garnish with raspberries and serve immediately.