In a medium bowl, combine the strawberries, blueberries, lemon zest, cornstarch, and sugar. Mix gently until well combined.
In a separate bowl, mix the brown sugar, flour, rolled oats, chopped pistachios, and cinnamon. Pour in the melted butter and stir until the mixture forms crumbly clusters.
Transfer the berry mixture to a baking dish. Evenly sprinkle the crumble topping over the fruit.
Bake in a preheated oven at 350°F (175°C) for 40 minutes, or until the topping is golden and crisp.
Spoon the crumble onto plates and serve warm with vanilla ice cream and fresh mint.