Let the blueberries and peaches thaw at room temperature while you make the simple syrup. Make simple syrup by combining the maple syrup and water in a small saucepan. Bring to a boil, then turn off the heat and let cool slightly.
Add the fruit, simple syrup, and lemon juice to a blender. Blend on a high setting - run the blender at least twice. If desired, serve immediately as a soft-serve sorbet.
Place the mixture in a freezer-safe ziplock bag. Place in the freezer for 4 hours, or overnight. Use your hands to massage the bag every hour (if desired, for less ice crystals to form).
When ready to serve, let thaw 2-3 minutes, then scoop. Garnish with fresh mint if desired, and enjoy!