Preheat oven to 375F (190C) and lightly grease a 9x9 or 11x7 glass baking dish² with butter. Set a side.
Combine sliced apples, brown sugar, lemon juice, flour, cinnamon, and salt in a large bowl. Give everything a good stir until all ingredients are combined and set aside while you prepare your crisp topping.
Prepare your topping by whisking together butter and vanilla extract. Set aside (you don't want the butter to be too hot when you add your sugar, so let it cool while you prepare your dry ingredients)
In a separate bowl, stir together flour, oats, sugar, baking soda, salt, and cinnamon.
Pour butter into flour mixture and use a fork to stir until mixture is combined and clumpy (don't over-stir, you want it to be crumbly, not paste-like). Set aside.
Give your apple mixture another good stir and pour evenly into your prepared pan.
Use your hands to drop your crumble mixture evenly overtop your apple filling.
Transfer to 375F (190C) oven and bake for 50 minutes or until topping is lightly golden brown and apples are tender when pierced with a fork. If topping is starting to become too dark and apples are still crisp, cover apple crisp with foil while continuing to bake.
Serve warm (preferably topped with ice cream!)
¹This was about 7 smallish apples for me. I recommend using Granny Smith apples as they are firm and tart. Using a sweeter apple may result in a sweeter crisp that cooks faster, so keep that in mind!²This recipe will work in a metal pan, but it will likely need to cook for less time so keep an eye on it and start checking sooner.See post for detailed instructions on storing apple crisp, making it ahead, or freezing it.