Preheat the oven to 350°F or 175°C. Lightly grease an 8-inch square baking dish so the apple filling does not stick around the edges as it bubbles.
Place the sliced apples in a large bowl. Add lemon juice, granulated sugar, brown sugar, flour, cinnamon, vanilla, and nutmeg, then toss until every slice is evenly coated.
Transfer the apple mixture to the prepared dish and spread it into an even layer. Press the fruit down gently so there are no large empty spaces between the slices.
In a separate bowl, combine the oats, flour, brown sugar, cinnamon, and salt. Pour in the melted butter and stir until the mixture forms moist crumbs.
Sprinkle the topping evenly over the apples without pressing it down firmly. Leaving the mixture loose helps the surface become crisp and golden while baking.
Bake for 38 to 42 minutes, or until the filling bubbles around the edges and the topping is browned. Let the crisp rest for 10 minutes before serving so the juices can thicken slightly.
