Prepare a 3-quart baking dish with butter. Add sliced apples to a large bowl and sprinkle with lemon juice. Add sugar, flour, cinnamon and toss to coat. Add blueberries and toss lightly. Add mixture to baking dish.
In a medium size bowl, combine oatmeal, brown sugar, pecans, cinnamon, salt and butter and mix together with fingers to form clumps of oatmeal mixture. Evenly top apple mixture with oatmeal mixture.
Bake for 35-45 minutes or until oatmeal topping starts to brown and apple mixture bubbles. Serve hot with ice cream or at room temperature.