Peel 4–5 medium apples, then cut the flesh off the core and slice very thinly.
In a mixing bowl, toss apples with brown sugar, cornstarch, cinnamon, vanilla, lemon juice, and a pinch of kosher salt until everything is evenly coated and starting to look a bit juicy.
Let the apples sit while you make the topping.
Make the topping
In a separate bowl, combine flour, brown sugar, and cinnamon; break up any brown sugar lumps with the flour.
Add the cold butter cubes and work them into the dry ingredients with your hands (or a pastry blender) until the pieces are about the size of peas and the mixture is evenly coated.
Assemble and bake
Spread the apple mixture evenly in a baking dish.
Sprinkle the crumb topping evenly over the apples so most of the surface is covered.
Bake at 350°F for about 40–50 minutes, starting to check around 40 minutes, until the apples are tender, the juices are bubbling, and the top is golden.
To serve
Let it cool just a bit, then serve warm with vanilla ice cream or whipped cream.