Set Up the Baking Dish and Oven:
Preheat the oven to 375°F (190°C).
An 8 × 8 or 9-inch square baking dish (or a comparable 2-quart casserole dish) should be lightly oiled.
Make the Apple Filling:
Combine the sliced apples, lemon juice, granulated sugar, 1 tablespoon flour, 1 teaspoon cinnamon, and nutmeg (if using) in a large basin. Toss until it coats the apples evenly.
Evenly distribute the apple mixture after pouring it into the baking dish.
Prepare the Crisp Top:
Mix 1 cup flour, brown sugar, oats, 21 teaspoons cinnamon, and salt in a separate medium bowl.
Combine the dry ingredients with the chilled, diced butter.
Cut the butter into the flour mixture with a pastry blender, two knives, or your fingertips until the mixture is sandy or resembles coarse crumbs, leaving some pea-sized bits of butter behind.
Assemble and Bake:
Evenly distribute the topping on top of the apple filling. Avoid applying pressure.
Bake for 45 to 55 minutes, or until the apple filling is soft and bubbling when pierced with a fork, and the topping is golden brown.
Cool and Serve:
Before serving, allow the crisp to cool on a wire rack for at least 15 to 20 minutes. The fluids might thicken as a result.
Serve warm, maybe with a dollop of whipped cream or a scoop of vanilla ice cream.