While waiting for the apples to cool, make crumb topping. Put the flour, oats, brown sugar, cinnamon, and salt in a food processor and pulse several times to mix. Scatter the butter over the top. Pulse repeatedly until the mixture resembles fine crumbs. Empty the crumbs into a large bowl then rub them together between your fingers until you have large, buttery crumbs. This step can also be done ahead of time with the mixture being placed in a storage bag and placed in the freezer.