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Preheat the oven to 375°F. To make the filling, toss the blueberries, rhubarb, apple, sugar, cornstarch, salt, and lemon juice together in a bowl. Transfer to a 6-cup baking dish (such as an 8-inch square, 11-x-7-inch rectangle, or a deep dish pie plate).
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To make the topping, combine the whole wheat flour, oatmeal, sugar, salt, and cinnamon in a food processor and pulse to blend. Add the butter or coconut oil and pulse until the mixture is well combined and resembles wet sand.
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Scatter the topping evenly over the filling and bake until the topping is golden brown and the fruit is bubbling, 45 to 55 minutes.