Make the topping: Rub together the flour, both sugars, the spice and salt in a medium bowl. Toss in the butter and nuts to coat, then pinch the butter into the dry ingredients until no floury bits remain. Add the oats, and gently rake and squeeze them through the buttery mixture to form peanut-size crumbles. Freeze while you prepare the apples. The crumb mixture can be frozen in an airtight container for up to 1 month.

Prepare the apples: Heat oven to 375 degrees. Use 1 tablespoon sugar for all sweet apples; 2 to 3 tablespoons for sweet-tart apples or a mix; and 4 tablespoons for all tart apples. Mix the sugar with the flour, spice and salt in a 10-inch cast-iron or other heavy ovenproof skillet.

If you’d like, peel the apples. Cut into 1/2-inch chunks, discarding the seeds and cores. Add to the skillet and drizzle with 1 tablespoon lemon juice for tart apples and 2 tablespoons for sweet-tart and sweet ones. Mix until evenly coated, then spread in an even layer. Crumble the frozen crisp mixture on top. (There will be gaps.)

Bake until the topping is golden brown, and the apples are tender and bubbling, 45 to 50 minutes. Cool for at least 15 minutes on a rack before serving hot, warm or at room temperature.