Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.
In a large bowl, toss the sliced apples with 2 tablespoons of flour, granulated sugar, cinnamon, nutmeg, and lemon juice. Ensure the apples are evenly coated. Pour the apple mixture into the prepared baking dish and spread it out evenly.
In a separate medium bowl, combine the rolled oats, 1/2 cup flour, brown sugar, 1/2 teaspoon cinnamon, and salt. Whisk to combine.
Add the cold, cubed butter to the oat mixture. Using your fingertips, a pastry blender, or two forks, cut the butter into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining. Don't overmix; you want some texture.
Evenly sprinkle the oat topping over the apples in the baking dish.
Bake for 35-45 minutes, or until the topping is golden brown and the apple filling is bubbly around the edges. If the topping starts to brown too quickly, you can loosely tent the dish with aluminum foil.
Let the apple crisp cool for at least 10-15 minutes before serving. This allows the filling to set slightly.