Peel, core, and slice the apples into thin pieces, then place them in a mixing bowl.
Toss the apple slices with lemon juice, cinnamon, and granulated sugar until evenly coated. This helps prevent browning and infuses flavor.
Transfer the seasoned apples into a buttered baking dish, spreading them in an even layer.
In a separate mixing bowl, combine the flour, oats, brown sugar, and salt. Mix briefly to distribute evenly.
Add the cold, cubed butter to the dry mixture. Using a pastry cutter or your fingers, work the butter into the mixture until it resembles coarse crumbs.
Spoon the crumbly topping evenly over the apples in the baking dish, covering the surface completely.
Place the baking dish in an oven preheated to 350°F (175°C) and bake for about 40 minutes, until bubbling around the edges and the topping is golden brown.
Remove from the oven and let cool slightly before serving. The crumble will be warm, with tender apples underneath a crisp topping.
