What you’ll need:

2 apples (Honeycrip, PinkLady, or GrannySmith are all fine choices)
1/4 tsp of cinnamon
1/2 tbs Sugar (or more to taste) 
1 tbs of flour
1 tbs of butter to grease the skillet
Crumble Topping: 
1/3 cup Butter (room temperature)
1/3 cup Flour
1/3 cup Sugar

Put it all together:

FIRST Preheat the oven to 350 degrees F, with the rack in the middle position. Grease your skillet using butter or cooking oil and set aside.

THEN Peel the apples, and cut them in 4 pieces removing the core. Thinly slice the apple pieces into 1/8″ chips. The thinner you can get them, the better. In a mixing bowl combine sliced apples, cinnamon, sugar and flour and mix until coated. 

NEXT  For the crumble, mix the flour and sugar together, and then “cut” the butter into the flour/sugar mixture using either two forks or a pastry cutter. Cold butter is fine to use here, and you can leave the mix chunky. 

FINISH Add your apple mixture to the skillet, pressing it down into all the nooks and cranny’s with your fingers, then cover the apples with the crumble. Bake 50-60 minutes, or until the crumble is golden and the apples are tender. Check them with a fork to ensure they