Prepare the raspberry puree: Add raspberries and sugar to a blender or food processor. Blend until completely smooth, about 30-45 seconds. The mixture should be thick but pourable.
Strain the puree: Pour the blended raspberry mixture through a fine-mesh strainer into a bowl, using a spatula to press and extract all the liquid while leaving seeds behind. This ensures a smooth, seed-free puree.
Add fresh lemon juice: Stir the freshly squeezed lemon juice into the strained raspberry puree until well combined. The mixture should brighten in color slightly.
Prepare glasses: If desired, wet the rims of champagne flutes with a lemon wedge and dip in sugar for a decorative touch. Let dry for 1-2 minutes to set.
Assemble the mimosas: Add 1-2 tablespoons of the raspberry-lemon mixture to each champagne flute. The exact amount depends on your preference for sweetness and fruit flavor intensity.
Pour the champagne: Slowly fill each glass with chilled champagne or prosecco, pouring at an angle to preserve the bubbles. The sparkling wine will naturally mix with the raspberry-lemon puree, creating a beautiful gradient effect.
Garnish and serve: Drop a fresh raspberry into each glass, add a small sprig of mint if using, and sprinkle with a pinch of lemon zest. Serve immediately while bubbles are still vibrant.