Preheat the oven to 350°F. Place the brown sugar, flour, oats, cinnamon, nutmeg, and salt in a large bowl, stir to combine.
Add the butter and stir it in so the pieces are coated in the flour mixture.
Work the mixture with your fingers or pastry cutter until it resembles wet clumpy sand. Any pieces of butter should be about the size of a pea or less. Place the bowl in the refrigerator until you are ready to assemble the crisp.
Cut the apples into 4ths then slice them about 1/4th of an inch thick, place them in a large bowl. Add the sugar, cornstarch, cinnamon, and nutmeg, toss to combine.
Add the filling to a 9 to 10-inch baking dish or cast-iron skillet.
Add the topping, it looks like a lot, but use it all.
Bake for 40-45 minutes until the topping is golden brown and you can see or hear the apples bubbling underneath.
Let cool for at least 20 minutes before serving.