Preheat your oven to 350°F. Grease a 9" x 13" rectangular casserole pan.
Toss the sliced apples with the remaining filling ingredients, and spread them evenly in the pan.
To make the topping, whisk together the flour, oats, salt, sugar, cinnamon, and baking powder.
Add the cold butter, working it in to make an unevenly crumbly mixture. Stir in the nuts, if you're using them.
Spread the topping over the apples in the pan.
Set the pan on a parchment- or foil-lined baking sheet to catch any potential drips.
Bake the apple crisp for about 60 minutes, till it's bubbling and the top is golden brown.
Remove the apple crisp from the oven, and allow it to cool for at least 20 minutes before serving. If you serve the apple crisp hot/warm, it may be quite soft. If you wait till it's completely cool, it'll firm up nicely.
Notes
Serving tips:Vanilla ice cream (or in my case Vanilla Caramel Swirl Gelato) is a tasty accompaniment.