Preheat oven to 350 degrees F.
Add ¼ cup all purpose flour, 2 Tablespoons chopped refrigerated butter, 1 Tablespoon sugar, a pinch of kosher salt, and a pinch of cinnamon to a medium sized mixing bowl or mini food processor. Using a fork, pastry cutter, or hand, cut butter into dry ingredients. Mix until a small pebble sandy texture.
Spray baking dish with cooking spray. Sprinkle ½ of crust mixture into baking dish (remaining mixture will be used for the topping). Press together with the smooth bottom of a glass or small measuring cup.
Bake 8-10 minutes until soft. May not be golden brown, that's fine.
While crust is baking, prepare the topping by adding 1 Tablespoon oats, ½ Tablespoon brown sugar, and ½ Tablespoon cubed refrigerated butter to remaining crust mixture. Mix together using a fork until a pebbly texture. Refrigerate until assembly.
Assemble the apple filing by adding ½ apple (sliced), 2 teaspoons all purpose flour, ½ Tablespoon sugar, ½ teaspoon lemon juice, ¼ teaspoon cinnamon, a pinch of kosher salt, and a pinch of nutmeg to a small-medium sized mixing bowl.
Toss until all apple slices are coated.
Remove crust from oven once done, evenly spread out apples over crust. Sprinkle remaining filling mixture over apples. Sprinkle topping over apples and spread it out to cover apples evenly.
Bake at 350 degrees for 25-30 minutes, or until topping is golden brown and apples are bubbling below.
Cool 10 minutes, or refrigerate. It's my favorite cold. Optional: drizzle caramel sauce over tart, and add a scoop of ice cream.