How to Make It
MAKE THE TOPPING In a medium bowl, stir the oats with the brown sugar, flour, walnuts, cinnamon and salt. Add the melted butter and stir until damp crumbs form. Let the topping stand at room temperature for 15 minutes.
MAKE THE FILLING Preheat the oven to 350°. Lightly brush a 9-inch square baking dish with butter. In a large bowl, toss the apples with the sugar, orange juice, cinnamon, nutmeg and salt. In a large skillet, melt the 2 tablespoons of butter over moderately high heat. Scrape the apple mixture into the skillet and cook over moderately high heat, stirring occasionally, until barely tender, about 5 minutes. Using a slotted spoon, spread the apples in the prepared baking dish. Cook the juices in the skillet until thick and syrupy, about 2 minutes. Scrape the juices over the apples and sprinkle with the topping.
Bake the crisp for 30 minutes, until the topping is browned and the filling is bubbling. Transfer to a wire rack and let stand for 15 minutes. Serve the crisp warm with vanilla ice cream.