Preheat the oven to 375 degrees F. Butter a 9-inch-square baking dish.
Toss the blackberries and blueberries in a large bowl with the maple syrup and tapioca. Transfer to the prepared baking dish.
Combine the flour, granulated sugar, brown sugar, nutmeg and salt in a food processor and pulse to combine. Add the hazelnuts, butter and vanilla; pulse until the nuts are roughly chopped. Sprinkle the topping over the berries, pinching it with your fingers to make small clumps.
Bake until the filling is bubbling and the topping is golden brown, about 1 hour. Let cool slightly before serving.
Photograph by Kana Okada
Recipe courtesy Food Network Magazine