Preheat oven to 375 F. Grease an 8 or 9 inch oven safe baking dish with butter. Set aside.
Start making the sauce by combining fruits, cornstarch, lemon juice, and brown sugar in a medium saucepot. Cook on medium heat until the mixture thickens.
This will happen fast, so make sure to stir the mixture regularly.
Remove from the heat and pour into the prepared baking dish, dot with butter cubes.
Place the flour, sugar, baking powder, and salt in a large bowl. Mix well with a spoon. or a fork.
Add butter (cut into cubes) into the flour mixture and gently stir (either with a spoon or your hands) until you have crumbs.
Transfer to the oven and bake for 25-30 minutes at 375F until topping is golden brown and filling bubbly.
Sprinkle a bit of brown sugar and old fashioned oats over the topping before baking. (Optional)
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