Preheat your oven to 350°F and grab an 8x8 inch baking dish (or similar size). Lightly grease it with a little butter so nothing sticks. This takes literally 2 minutes but makes cleanup so much easier afterward. Trust me on this.

Slice your 2 cups of apples into thin, even pieces — aim for about 1/4 inch thick if possible. Toss them gently into your prepared baking dish and spread them out in a single layer. You'll want them fairly even so they cook at the same rate.

In a separate bowl, mix together the 1/2 cup rolled oats, 1/2 cup all-purpose flour, 1/2 cup chopped walnuts, 1 teaspoon cinnamon, 1/4 teaspoon nutmeg, and 1/4 teaspoon salt. Stir this dry mixture really well so the spices spread throughout evenly. This is where all the flavor magic happens.

Cut your 1/2 cup unsalted butter into small cubes and add it to the oat mixture. Use a fork or your fingertips to work it together until you get pea-sized clumps throughout. This won't look smooth — and that's exactly what you want.

Combine the 1/2 cup granulated sugar and 1/4 cup brown sugar in a small bowl. Sprinkle this sugar blend over the apples in your baking dish before adding the crumble topping. The sugar draws out moisture from the fruit and helps create that jammy filling.

Spread your crumble mixture evenly over the apples, pressing down gently but not too hard. You want it packed enough to hold together but loose enough that it'll get crispy. Don't compact it super tight or you'll end up with a dense cake layer instead of that golden crisp texture.

Bake for 40 minutes until the crumble topping turns golden brown and you can see the apple filling starting to bubble around the edges. The kitchen smells absolutely amazing at this point. Let it cool for 10 minutes before serving so the filling sets up a little bit.