Add the blackberries to a 9x13-inch baking pan or a similar-sized ovenproof dish. Sprinkle the cornstarch and sugar over the mixture and toss to combine. Set aside.
In a large mixing bowl, add the flour and brown sugar, then mix to combine.
Add the cold, cubed butter to the mixing bowl. Use your fingertips to gently rub the butter into the flour and sugar mixture until it resembles large crumbs.
Tip the oats into the mixing bowl and stir to combine with the rest of the ingredients.
Use a spoon to scatter the crisp topping on top of the prepared fruit. Do your best to evenly cover all the fruit. You do not need to press the topping down.
Bake on the middle rack of the preheated oven for 35 minutes, or until the top of the crisp looks golden brown.
Remove the pan from the oven and leave to cool for a few minutes before serving.
Enjoy while still warm with a scoop of vanilla ice cream. Garnish with a few fresh blackberries.
Notes
You can use either fresh or frozen blackberries in this recipe.
If using frozen blackberries, you do not need to thaw them before using.
Cold butter will give the crisp the best texture.
You can substitute the blackberries with any other berry or a mixture of several, keeping to 24 ounces in total as listed in the recipe.
Blackberry crisp can be enjoyed fresh from the oven or as a chilled leftover.
Store in an airtight container and refrigerate where it will stay fresh for at least 4 days.