Preheat your oven to 375°F (190°C). Grease the baking dish with a little butter or non-stick spray.
Thinly slice the apples and spread them evenly across the bottom of the dish. Sprinkle with granulated sugar, cinnamon, and nutmeg if using, then toss gently to coat and let them soften slightly as they bake.
In a mixing bowl, combine the flour, brown sugar, and salt. Add the cold butter pieces and use a pastry cutter or your fingers to blend until the mixture resembles coarse crumbs.
Sprinkle the crumbly streusel evenly over the apples, covering the surface completely. This layer will become crisp and golden as it bakes.
Place the dish in the preheated oven and bake for about 40 minutes, until the top is golden brown and the apples are bubbling along the edges.
Remove from the oven and let cool slightly. The casserole should look layered with tender apple slices beneath a crisp, crumbly topping.