To the bowl of a food processor, fitted with the chopping blade, add the flour, brown sugar, cinnamon, nutmeg, and salt: pulse a couple of times just to break up the brown sugar and combine the ingredients.
Add the butter and pulse until the mixture resembles coarse meal; about 10 pulses.
Add the nuts and pulse just until the crisp topping starts to clump together.
Remove from the food processor and transfer to a bowl.
Cover with plastic wrap and refrigerate while you make the filling.