Preheat oven to 375°.
Place the fruit, cornstarch, sugar and lemon juice in a medium bowl and stir gently to mix. Set aside while preparing topping.
Place the oats, brown sugar, flour, cinnamon, butter and nuts in a medium bowl. Using a pastry cutter or your clean hands, mix the ingredients together until they form coarse crumbs. You will still see small chunks of butter when the mixture is done. Note: Crumble mixture can be covered tightly and frozen for up to 3 months.
Place the fruit mixture in a lightly buttered 8-inch baking dish or 6 individual 1-cup dishes. Top the fruit with the crumb mixture and bake 30–35 minutes, until the fruit is bubbling and topping is lightly browned. Cool 10 minutes before serving.