Preheat the oven to 350F degrees. Grease a 2-quart baking dish.
In a medium bowl stir together the flour, brown sugar, oats, cinnamon & salt.
Add in the butter and cut it in using a pastry cutter or rub the butter into the dry ingredients using your fingers. At the end it should be crumbly in texture, but no large chunks of butter should remain. Place in the fridge as you make the filling.
Peel and slice the apples into thin slices about 1/8 - 1/4 inch thick.
Whisk together the melted butter, flour, water, sugar, cinnamon & nutmeg.
Pour the mixture over the sliced apples and toss to coat.
Spoon the apples into the greased pan, then crumble the oatmeal crumble overtop.
Bake in the preheated oven for 30-40 minutes, or until the top is golden brown and you can see the apples bubbling around the edges of the pan.
Remove from the oven and cool for at least 10 minutes before serving.