Preheat oven to 400 degrees (F).
Place the blackberries and half the sugar in a pan. If using fresh berries, add 2 tablespoons water, but no water is needed if using frozen berries. Heat while stirring the mixture until the sugar dissolves, then add the cornstarch. Stir over medium heat for 3 to 5 minutes until thickened.
Pour the blackberry mixture into a 10-inch oven-safe dish, then set aside while preparing the topping.
For the cobbler topping, mix together sifted flour, rolled oats, cornmeal, baking powder, cinnamon, cardamom, salt, and the remaining sugar. Stir in the lemon juice, melted coconut oil, and milk. Mix lightly, but it should be clumpy rather than smooth.
Add the crumbly topping on top of the berries and bake 400 degrees for about 30 minutes or until the fruit bubbles up and the crumble topping is baked through.
Serve warm.
*Jaggery is an unprocessed sugar that's used widely in India and can be found at Indian grocery stores or online. It's a bit less sweet than brown sugar, with a deeper molasses flavor.
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