In a small saucepan, combine the raspberries and sugar over medium heat. Cook until the berries break down, about 5 minutes.
Press the mixture through a fine sieve to make a smooth raspberry puree. Let it cool.
In a pitcher, stir together the raspberry puree and fresh lemon juice.
Pour in the sparkling water and stir gently to combine.
Fill glasses with ice, pour in the lemonade, and garnish with extra raspberries and lemon slices.