Arrange the watermelon, strawberries, and mixed berries in a single layer on a parchment-lined baking sheet. Freeze for at least 2 to 3 hours, until the fruit is completely solid.

Transfer the frozen fruit to a food processor. Add the lemon juice and maple syrup. Process until the mixture becomes smooth and creamy, pausing to scrape down the sides as needed.

If the texture is too thick to blend, add cold water one tablespoon at a time, just enough to help the mixture move.

Serve immediately for a soft sorbet, or transfer to an airtight container and freeze for 1 to 2 hours until scoopable. Garnish with mint leaves, if using.