In a small saucepan, combine the sugar and water. Heat over medium heat, stirring until the sugar is completely dissolved. Add the mint leaves, bring to a simmer, and then remove from heat. Let the mint steep in the syrup for 30 minutes to infuse the flavor.

After 30 minutes, strain the mint syrup through a fine-mesh sieve into a bowl, discarding the mint leaves. This step ensures a smooth sorbet without any bits of mint.

In a food processor or blender, combine the mixed berries and lemon juice. Blend until completely smooth.

Pour the mint syrup into the berry puree and stir well to combine. Cover the mixture and chill in the refrigerator for at least 2 hours. This allows the flavors to meld together and the mixture to cool thoroughly.

If you have an ice cream maker, pour the chilled mixture into the ice cream maker and churn according to the manufacturer's instructions. This usually takes about 20-25 minutes, or until the sorbet reaches a soft-serve consistency.

If you don’t have an ice cream maker, pour the chilled mixture into a freezer-safe container. Freeze for about 3-4 hours, stirring every hour to break up ice crystals. This will help create a smoother texture.

Once the sorbet has reached your desired consistency (either from the ice cream maker or the freezer), it’s ready to serve!