Cut the apples into small pieces. Aim for something between 8 slices around cut into 4 pieces each and 12 slices cut into 6 pieces each. Throw out the core.
Mix the apple pieces and white flour together. The flour will absorb excess juice from the apples.
Seperately, mix the whole wheat flour and salt.
Cut the butter into small pieces. The smaller the pieces you cut it into, the easier the next step will be.
Cut the butter into the dry ingredients using a pastry blender. Keep doing this until the butter is the size of small peas.
Mix the oats and sugar into the flour mixture.
Stir the water into the topping mixture.
Spread the apple pieces out in the bottom of the pan. They should fill it most of the way to the top, but they will shrink when cooked. You don't need to grease the pan, but nothing bad happens if you do.
Distribute the topping mixture over the apples. It will be too stiff and sticky to spread, so you will probably have to drop it in little chunks. It will get harder when cooked.
Bake at 375°F (190°C) for 35 minutes.
I previously said to cut the apples into 16-24 slices. That was an error and difficult to do.