Preheat your oven to 350°F. Grease a 9" x 9" square cake pan, or similar-size casserole pan., Slice the apples about 1/4" thick. Toss them with the remaining filling ingredients, and spread them 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 cookie sheet, to catch any potential drips. Bake the crisp for about 60 minutes, till it's bubbling and the top is golden brown., Remove it from the oven, and allow it to cool for at least 20 minutes before serving. If you serve the crisp hot/warm, it may be quite soft. If you wait till it's completely cool, it'll firm up nicely. A good compromise is to wait for it to cool completely, then reheat individual portions briefly in the microwave. Vanilla ice cream is a tasty accompaniment.