Preheat the oven to 325°F with a rack in the center. Line an 8" square pan with a parchment sling and lightly grease the parchment.
To make the crust: In a medium bowl or the bowl of a stand mixer, beat the butter and brown sugar on medium speed until smooth, about 30 seconds. Add flour and salt, beating on low speed until crumbly. Press the mixture into an even layer in the prepared pan.
Prick the dough all over with a fork.
Bake the crust for 15-20 minutes until lightly golden. Remove, increase oven temperature to 350°F, and allow crust to cool slightly.
To make the filling: In a large bowl, combine apples and lemon juice. In a skillet, melt butter over medium-high heat, add spice, then stir.
Add apples to the skillet and stir to coat. Cook for about 5 minutes until they begin to soften.
Sprinkle sugar over apples and stir, cooking for 6-10 minutes until most liquid evaporates. Remove from heat and allow to cool.
To make the topping: In a medium bowl, combine flour, brown sugar, and spice. Add butter, working it into the mixture until crumbly.
To assemble and bake: Spread the apple filling over the crust evenly, then top with the crumbly topping.
Bake for 35-40 minutes until topping is light golden. Allow to cool completely before slicing and serving.
Store leftovers wrapped at room temperature for several days, or freeze for longer storage.