For the topping:
¾ cup all-purpose flour
¾ cup old-fashioned oats
½ cup packed dark brown sugar
1 stick (8 tablespoons) unsalted butter, cold and cut into 4 pieces
1 teaspoon cinnamon
¼ teaspoon nutmeg
For the filling:
5 cups (5-6 medium) tart apples, peeled, cored and cut into thick slices
2 tablespoons sugar
juice of ½ lemon
Preheat oven to 375ºF. Lightly butter a 9-inch square dish.
Put all the topping ingredients in a food processor and pulse until the mixture forms big curds, about 1 minute. (You can make the topping up to 3 days ahead and refrigerate it in an airtight bag.)
Toss all the filling ingredients together in a large bowl. Transfer the apple mixture to the prepared dish. Layer the topping evenly over the apples. Bake the crisp for 55 minutes, or until the topping is golden and the fruit juices are bubbling up around it. Transfer the dish to a wire rack and let it rest for at least 15 minutes before serving.