Place a small pot over medium heat and add the blueberries, blackberries, raspberries, granulated sugar and lemon zest and juice. Cook until the sugar is dissolved and the berries have released their juices, about 6 minutes.
Meanwhile make the granola. Place a large nonstick skillet over medium-high heat. Add the oats and almonds and toast until light golden, 2 to 3 minutes. Remove from the skillet and reserve. Reduce the heat to medium, then add the butter to the skillet. Once the butter is melted, add the maple syrup, brown sugar, cinnamon and salt. Bring to a boil and cook for 1 minute, until you have a thin syrup. Add the toasted ingredients back to the skillet. Cook, stirring and tossing the ingredients, for 1 minute. Transfer to a plate and set aside.
Make a slurry by stirring the cornstarch into 2 tablespoons of water. Stir into the berry mixture and allow to thicken, about 1 minute.
Portion the berry mixture into 4 serving bowls. Top liberally with the oat and almond mix. Add 1 scoop of ice cream to each and garnish with a little more of the berry mix, a little more granola and some mint, then serve warm.