Preheat the oven to 175 °C (350 °F).
In a 9x13-inch baking dish, arrange the sliced apples. Mix the white sugar, 1 tbsp flour, and ground cinnamon; sprinkle over the apples. Pour water evenly over them.
In a separate bowl, combine oats, 1 cup flour, brown sugar, baking powder, baking soda, walnuts, and melted butter. Crumble the mixture evenly over the apple mixture.
Bake until golden and crisp on top, about 45 minutes.
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