Preheat oven to 375°F. Grease a 6 or 8 cup baking dish and set aside.
Peel apples, and cut into slices. Add to greased baking dish.
Sprinkle with sugar and flour, tossing to coat. Set aside.
Make the topping in a medium bow. Add oats, brown sugar, butter, flour, and apple pie spice and mix 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 to your liking. If the top starts to brown but the fruit is not ready, loosely tent 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.
Allow to cool for 5 minutes to thicken the fruit mixture before serving. Serve with whipped cream or ice cream if desired.
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.