Preheat the oven to 400 degrees F.
In a bowl combine the flour and diced butter in a food processor.
Add the sugar and iced water and pulse until the dough forms a ball.
Cover with plastic wrap and store in the fridge while you prepare the filling.
Core and peel the apples and slice into wedges
Cut the wedges into small dice and add to a bowl with the brown sugar, cinnamon and salt.
Toss well and let rest while you make the slab pie.
Take the dough from the fridge and roll it out large enough to fill a 15x10x1 inch baking pan making sure to leave a slight overhang of dough.
Place the dough sheet into the pan and press down around the edges making sure to fit it in nicely.
Trim any dough around the edges.
Take the apple filling and place it into the pie crust spreading the apples out to fit evenly and covering any gaps you can see.
In a separate bowl make the topping by combining the flour, grated butter, both sugars, oats and cinnamon.
Sprinkle the topping evenly over the apples and bake in the oven for about 45-50 minutes
Once the slab pie is baked, make the glaze by whisking the powdered sugar, milk and vanilla together until smooth.
Use a spoon to drizzle the glaze over the apple slab pie. Slice into small squares and serve.