For the shortbread base:

1. With a wooden spoon or electric hand mixer on low, cream butter with sugar until smooth.
2. Add salt and flour in two parts, combining well. Do not overmix. It will be crumbly.
3. Line an 8-inch square pan with aluminum foil or parchment paper and pat the dough evenly on the bottom. Lightly flour the tips of your fingers if needed to do this. Be sure all surface is covered as evenly as possible.
4. Prick the bottom with a fork and freeze for 30 minutes.
5. Preheat the oven to 350ºF / 180ºC.
6. Bake for 15 to 20 minutes, until it’s dry and starts to color lightly. Reserve.

For the crumble:

1. Mix flour and sugars in a medium bowl.
2. Add butter pieces and integrate them with a fork, a pastry cutter, or your hands (my choice).
3. Keep refrigerated.

For the apples:

1. Peel and core the washed apples.
2. Cut in half and then in half again to have 4 wedges. Remove the seeds together with the hard center parts.
3. Slice with a kitchen knife and transfer to a large bowl.
4. Sprinkle sugar and cinnamon on top and drizzle with lemon juice. 

To assemble:

1. Top the crust with the apple mixture, spreading to cover it all. It will be a lot of apples, but they will shrink during baking. 
2. Add the crumble evenly on top of the apples.
3. Bake for about 40 minutes, until the top is golden and the apples can be easily pierced with a knife.
4. Let cool on a wire rack.
5. Serve at room temperature or warm with a scoop of ice cream or whipped cream.