Line or grease a square tin and preheat the oven to 350 F.
Add 2 tablespoons of water to the flax and leave to one side. While that is thickening, sift together the flour, salt, baking powder and tablespoon of sugar, then rub in the oil until the mixture resembles breadcrumbs.
Add the flax mixture and knead until it becomes a firm but pliable dough. Put into the tin and press to cover the bottom evenly.
Brush a thin layer of apple sauce onto the dough, then cut the apples into thin slices and layer them on top. Use the rest of the apple sauce to cover the apple slices.
Mix the oats, sugar and cinnamon and sprinkle on top of the apples.
Bake for 1 hour.
Leave in the tin for 10 minutes, then carefully transfer to a cooling rack.
Can be eaten warm with a big scoop of coconut milk ice cream or left to cool.
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