In a blender, combine the fresh raspberries and lemon juice. Blend until smooth, then strain the mixture through a fine mesh sieve into a large pitcher to remove the seeds.

Add the granulated sugar to the raspberry-lemon juice mixture and stir until the sugar is completely dissolved.

Pour in the cold water and mix well. Taste and adjust sweetness by adding more sugar if desired.

Just before serving, add the sparkling water to the pitcher for a fizzy touch. Stir gently to combine.

Fill glasses with ice cubes and pour the raspberry lemonade over the ice. Garnish with lemon slices and fresh raspberries.