Preheat oven to 350 degrees F. Grease a 9-inch square pan, or other 2½-quart baking dish.
Prepare the Apples: Toss the apples with the apple cider, brown sugar, flour, melted butter, boiled cider (if using), cinnamon, nutmeg, ginger, and salt, and spread the mixture in the prepared pan.
Make the Topping: In a large bowl, whisk together the flour, oats, salt, sugar, cinnamon, and baking powder. Add the cold butter and work it into the flour mixture using a pastry blender, forks, or your fingers, until mixture resembles large, uneven crumbs. Toss in the chopped nuts (if using). Spread the topping evenly over the apples in the pan.
Bake the Apple Crisp: Line a rimmed baking pan with parchment paper or foil and place the pan with the apple crisp on top (to catch any potential drips). Bake the crisp until it's bubbling and the top is golden brown, about 55 to 60 minutes. Allow to cool for at least 20 minutes before serving. Leftovers can be stored, covered, at room temperature for up to 3 days or in the refrigerator for up to 1 week.
Use your favorite type of apples or a combination of varieties.
You can substitute apple juice, liquor of your choice, or water for the apple cider.
You can substitute 1½ teaspoons apple pie spice for the cinnamon, nutmeg and ginger.
To make in a 9x13-inch pan, increase the ingredients by 1.5.