Preheat oven to 375 F.
Lightly grease a 4-quart baking dish or 12 ramekin.
To prepare topping: in a medium-sized bowl, combine oats, ½ cup of the flour, ½ cup of the sugar and cinnamon.
Between fingers, rub butter with dry ingredients to form coarse crumbs.
Stir in nuts.
To prepare filling: in a separate, large bowl, toss blueberries and apples with the remaining ½ cup sugar and ½ cup flour.
Pour blueberry-apple mixture into prepared baking dish.
Sprinkle the topping evening over the fruit.
Bake until apples are tender and topping is golden, about 50 minutes if using a large baking dish or 20-30 minutes using ramekins.
Serve warm, with yogurt topping, if desired.