Preheat your oven to 350°F before you start anything else, since apple crisp bakes best in a fully heated oven. Lightly grease your 2-quart baking dish with butter or nonstick spray so the filling doesn't stick to the edges. I always do this step first so I'm not scrambling once my apples are sliced and ready to go.
Peel and slice your apples into even, thin pieces so they cook at the same rate. In a large bowl, toss the apple slices with lemon juice, granulated sugar, cinnamon, and nutmeg until every slice is coated. Let the mixture sit for a few minutes while you make the topping, since this helps the apples release a little juice.
In a separate medium bowl, stir together the oats, flour, brown sugar, and salt until well combined. Pour the melted butter over the dry mixture and stir with a fork or spatula until it turns into a thick, crumbly texture. You want it clumpy, not runny, so add a touch more flour if it feels too wet.
Pour the apple filling into your prepared baking dish and spread it into an even layer. Sprinkle the oat topping evenly over the apples, covering as much of the surface as you can. Don't press it down too firmly, since a loose topping bakes up crispier than a packed one.
Place the dish in your preheated oven and bake for 35 to 40 minutes, until the topping is deep golden brown. You'll know it's ready when you see the apple filling bubbling up around the edges of the dish. If the topping browns too quickly, loosely tent it with foil for the last few minutes.
Let the apple crisp rest for about 10 minutes before serving so the filling has time to set slightly. This short cooling period also means you won't burn your tongue on that molten apple filling. Serve it warm, on its own or with your favorite topping.