1. Heat the oven to 400 degrees. Butter a 2-quart casserole.
2. Combine blueberries, sugar, cornstarch, lemon juice and cinnamon in a large bowl. Mix well. Pour mixture into casserole and cover loosely with foil. Bake 15 minutes.
3. Lower oven temperature to 375 degrees and bake 20 more minutes.
4. Meanwhile, combine flour, oats, walnuts, cinnamon and butter in a large bowl. Mix with your hands until the texture becomes crumbly.
5. Remove casserole from the oven. Discard foil. Sprinkle topping over the hot blueberries. Return casserole to the oven and bake 30 minutes or until the topping is brown and the filling is bubbling. Cool about 1 hour (to just warm) before serving. If desired, top each serving with a scoop of ice cream.
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