Preheat your oven to 350°F (175°C). Lightly grease a 9x9-inch baking dish or similar-sized casserole dish with butter or nonstick spray to keep the apples from sticking and make serving easier.
Peel the apples, cut them into quarters, remove the cores, and slice them into roughly 1/4-inch thick pieces. Aim for even slices so they bake evenly, but don’t worry if a few are thicker.
6 medium apples
In a large bowl, combine the sliced apples, lemon juice, 1/3 cup granulated sugar, 2 tablespoons light brown sugar, 2 tablespoons flour, 1 teaspoon cinnamon, nutmeg (if using), 1/4 teaspoon salt, and vanilla extract. Toss well until all the slices are evenly coated and no dry flour remains.
6 medium apples, 2 tablespoons lemon juice, 1/3 cup granulated sugar, 2 tablespoons light brown sugar, 2 tablespoons all-purpose flour, 1 teaspoon ground cinnamon, 1/4 teaspoon ground nutmeg, 1/4 teaspoon salt, 1 teaspoon vanilla extract
Pour the apple mixture into the prepared baking dish and spread into an even layer, tucking in any pieces that stick up so they don’t burn. Any sugar-spice mixture at the bottom of the bowl should be scraped in as well.
6 medium apples
In another large bowl, stir together the oats, 3/4 cup flour, 1/2 cup light brown sugar, 1/4 cup granulated sugar, 1/2 teaspoon cinnamon, and 1/4 teaspoon salt. Use a fork or whisk to break up any clumps of brown sugar.
1 cup old-fashioned rolled oats, 3/4 cup all-purpose flour, 1/2 cup light brown sugar, 1/4 cup granulated sugar, 1/2 teaspoon ground cinnamon, 1/4 teaspoon salt
Add the cold, cubed unsalted butter to the dry topping mixture. Use a pastry cutter, two knives, or your fingertips to cut the butter into the mixture until it resembles coarse crumbs with pea-sized bits of butter. Do not overwork; a few larger chunks are fine.
1/2 cup unsalted butter
Stir in the chopped pecans or walnuts, if using. Gently pinch portions of the mixture in your hands to create a few chunky clusters, which will bake up into extra-crunchy bites.
1/2 cup pecans or walnuts
Sprinkle the oat topping evenly over the apples, covering them as much as possible without pressing down firmly. Leave some texture and cragginess on top so it crisps nicely.
1 cup old-fashioned rolled oats, 3/4 cup all-purpose flour, 1/2 cup light brown sugar, 1/4 cup granulated sugar, 1/2 teaspoon ground cinnamon, 1/4 teaspoon salt, 1/2 cup unsalted butter, 1/2 cup pecans or walnuts
Bake in the preheated oven for 40–45 minutes, or until the topping is deep golden brown and you can see the apple juices bubbling around the edges. If the topping is browning too quickly around the 30-minute mark, loosely tent with foil and continue baking.
Let the apple crunch rest for 10–15 minutes before serving to allow the juices to thicken. Serve warm, ideally with vanilla ice cream, whipped cream, or a drizzle of heavy cream.