Preheat oven to 375°F. Grease a 6 or 8 cup baking dish and set aside.
Peel apples, and cut into slices.
Place apples into baking dish. Sprinkle with sugar and flour, tossing to coat. Set aside.
Make the topping by mixing together the oats, brown sugar, butter, flour, and apple pie spice with a pastry cutter or fork. Once fully mixed, stir in nuts if using. Spoon topping on top of fruit, pressing down lightly.
Bake 30-35 minutes, or until bubbling on the edges and apples are cooked through. If it starts to brown but the fruit is not ready, cover loosely with foil or parchment and bake for 5 more minutes. If still not ready, check after 3 minutes until it is cooked to your liking.
Let cool for 5 minutes to thicken the fruit mixture before serving.
Notes
Cover leftovers and refrigerate for up to 3 days, or freeze for up to 3 weeks.
To make a day ahead, assemble the apple and crisp mixtures in separate bowls and refrigerate for up to 24 hours.
To make ahead and freeze, assemble the apple crisp in a baking dish, wrap in foil, then plastic, and freeze for up to 3 months. Bake at 350°F and add 10-15 minutes to the baking time.