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

Stir in the honey or agave syrup until fully dissolved. Taste and adjust sweetness if necessary.

Add the sparkling water to the berry-lemon mixture and gently stir to combine.

Fill glasses with ice cubes and pour the berry lemonade spritz over the ice.

Garnish each glass with fresh mint leaves and a lemon slice before serving.