Preheat oven to 425ºF. Combine berries, sugar, vanilla and cornstarch in a large bowl. Stir to combine, mashing some berries with back of spoon to release some juice.
In a separate bowl, combine walnuts, coconut, oats, flour, butter and salt. Mix with your fingers until a dough forms.
Spread berry mixture in an 8-inch square baking pan. Scatter small clumps of dough over berries. Bake until berries are bubbling and top is golden, about 30 minutes. Serve warm, with ice cream, if desired.