Preheat the oven to 350 degrees F (175 degrees C).
In a baking dish, mix sliced strawberries, blueberries, raspberries, vanilla extract, lemon zest, granulated sugar, and cornstarch.
In a large bowl, combine brown sugar, flour, oats, and kosher salt.
Cut cold unsalted butter into small pieces and add it to the bowl. Use fingers or a pastry cutter to blend the butter into the dry ingredients until a crumbly texture forms.
Sprinkle the crumble mixture evenly over the berries in the baking dish.
Bake for about 30 minutes, or until the crumble topping is golden brown and the berries are soft and slightly juicy.
Remove from the oven and let it cool slightly.
Serve the mixed berry crumble warm with a scoop of ice cream on top, if desired.
Store any leftovers in an airtight container in the refrigerator for up to three days.