Make the crumble. In a large bowl, mix together flour, brown sugar, butter and oats with a pastry blender. In a 2 quart baking dish (8x8”, 11x7” or 9” circle), press half of the mixture into the bottom of the dish, reserving the other half to sprinkle over the top.
Make the filling. In a separate bowl, toss together blackberries, white sugar, cornstarch, lemon juice, and lemon zest until the blackberries are completely coated and the sugar and cornstarch are almost completely dissolved. Add blackberries to the top of the crumble layer, then sprinkle the reserved crumble on top.
Bake until the crumble starts to brown and the blackberry mixture is bubbly, about 40-45 minutes. Let cool for at least 30 minutes before serving. We love it served with a big scoop of vanilla ice cream.
Notes
You can substitute fresh berries for frozen blackberries straight from the freezer (do not thaw). Frozen berries release more liquid as they bake, so the filling may need a few extra minutes in the oven to thicken and bubble properly.