Heat your oven to 350°F and grab a 9x13 inch baking dish. Slice your 6 apples into thin, even pieces—you'll want them kinda thin so they bake through. This takes about 5 minutes, and honestly, I use a mandoline to speed it up but a sharp knife works fine.

Toss your sliced apples with the granulated sugar, brown sugar, cinnamon, nutmeg, and lemon juice in a large bowl. Mix it really well so every apple slice gets coated with the spice mixture. Pour everything into your baking dish and spread it evenly.

Cut your cold butter into small cubes and put them in another bowl. Add the flour, rolled oats, chopped walnuts, and salt. Use your fingers to work the butter into the dry ingredients until it looks like coarse sand with pea-sized chunks.

The crumble should feel kinda crumbly but hold together slightly when you squeeze it. Don't overmix this part or you'll end up with a dense topping instead of that crispy texture you want. Mine never looks perfectly even and that's totally fine.

Spread your crumble mixture evenly over the apple layer. Don't pack it down—let it sit loose so air can get in and make it golden and crispy. You'll hear it start to sizzle around minute 30.

Bake for 45 minutes until the crumble top turns golden brown and the apple filling bubbles around the edges. If your topping browning too fast, loosely tent it with foil after 35 minutes. My oven always browns faster on one side so I rotate the pan halfway through.

Let it cool for at least 10 minutes before serving—I know it's hard to wait but cutting into it while it's piping hot makes it fall apart. The filling sets up as it cools, so you'll actually get those nice apple crisp recipe slices instead of mush.