Set your oven to 350°F (175°C). Lightly grease a 9×9-inch baking dish (or something similar in size). A little butter or nonstick spray does the trick.
In a large bowl, toss your peeled and sliced apples with both sugars, cinnamon, nutmeg, allspice, lemon juice, and cornstarch. Mix until the apples are nicely coated with that cozy spice blend.Tip: Don’t skip the lemon juice it brightens everything up and balances the sweetness! Spread the mixture evenly into your greased baking dish.
In another bowl, combine oats, flour, both sugars, cinnamon, and salt. Add in the cold, cubed butter. Use a pastry cutter or your fingers (my go-to method) to work the butter into the mixture until it forms coarse crumbs. Stir in the chopped nuts if you’re using them.
Sprinkle that glorious topping evenly over your apple mixture. Don’t worry if it’s not perfect this is rustic baking at its finest.
Pop it in the oven and bake for 35–40 minutes, until the topping is golden brown and the apples are bubbling and tender when pierced with a fork.
Once it’s out of the oven, let it rest for 5–10 minutes. This helps the juices thicken a bit and makes it easier to serve.
Scoop into bowls, top with a big ol’ scoop of vanilla ice cream, and drizzle with caramel sauce if you’re feeling fancy (or if it’s just been that kind of day).