Preheat oven to 350°F. Lightly grease or butter a 6" x 10" baking pan (or use a 9" x 13" for easier handling). This takes 10 minutes. Checkpoint: Oven should be fully preheated and pan ready.

Wash, peel, core, and slice apples into thin, even slices (about ¼-inch thick). Layer them evenly in the bottom of the prepared pan. This takes 5–8 minutes. You should have a full, compact layer of apple slices with no gaps. Checkpoint: Apples are evenly spread and filling the pan.

In a medium mixing bowl, add flour, salt, sugar, unbeaten egg, and baking powder. Using two knives or a pastry blender, cut the ingredients together with quick, crosswise motions for 2–3 minutes until the mixture looks like coarse breadcrumbs or wet sand. The egg will coat the flour, but don't overmix. Checkpoint: Mixture is crumbly with no large clumps and resembles damp sand.

Spread the crumbly topping evenly over the apples using the back of a spoon or your hands (lightly dampened fingers work best). Cover the apples completely but don't press down hard—leave it loose and crumbly. This takes 2–3 minutes. Checkpoint: All apples are covered with an even layer of crumble.

Pour ½ cup melted butter slowly over the entire top layer, letting it soak into the crumble. This takes 1 minute. The butter will help create the crispy topping. Checkpoint: Butter is distributed across the top.

Sprinkle 1–2 teaspoons of cinnamon evenly over the buttered top. Use a light hand or sift it on for even coverage. This takes 1 minute. The cinnamon should be visible but not clumpy. Checkpoint: Cinnamon is evenly distributed.

Place the pan on the middle oven rack. Bake at 350°F for 30 minutes. At 15 minutes, check that the apples are starting to bubble gently at the edges—this tells you they're cooking. Checkpoint: At 15 minutes, you should see light bubbling around the edges.

Bake for the final 15 minutes until the topping is golden brown and crispy (not pale), and the apple juices are bubbling slightly around the edges. The topping should sound crispy if you tap it lightly. Total baking time is 30 minutes. Checkpoint: Topping is golden brown, feels crispy to the touch, and apples are fork-tender.

Remove the pan from the oven carefully using oven mitts. Set it on a heat-safe surface or cooling rack. Let cool for 5–10 minutes before serving. This resting time allows the apples to set slightly and makes it easier to serve. Checkpoint: Crisp has cooled enough to handle safely but is still warm.