Preheat the oven to 375°F. Grease a 9 x 13-inch pan with the butter.

Make the filling: In a large mixing bowl, stir together the apples, flour, brown sugar, cinnamon, and salt until the apples are evenly coated. Dump the apples into the prepared pan and bake for 30 minutes, or until the apples are beginning to soften. (If you’re scaling, bake less time for less fruit, more time for more fruit.)

Make the topping: In the same bowl, stir together the flour, oats, nuts, brown sugar, cinnamon, and salt. Drizzle the butter on top, then mix until no bits of flour remain and the mixture holds together like a dry cookie dough.

Add the topping and finish baking: Give the apples a brief, gentle stir, then scatter the topping evenly over the apples. Bake for 25 to 35 minutes more, or until the filling bubbles at the corners and the topping is golden brown. Serve warm or at room temperature.

Store any leftover crisp at room temperature, covered, up to a day, or refrigerated for up to 3 days.