Preheat your oven to 350 degrees Fahrenheit and lightly grease your 9×9-inch baking dish. This ensures the crisp does not stick and bakes evenly.
In a large bowl, combine sliced apples with sugars, cinnamon, nutmeg, lemon juice, and flour. Toss until every apple slice is evenly coated in the fragrant mixture, which will help thicken the juices while baking.
Transfer the apple mixture to the prepared baking dish and spread it evenly. This layer will form the sweet, juicy base of your apple crisp.
In a separate bowl, mix the oats, flour, brown sugar, cinnamon, and salt. Stir in melted butter until the mixture becomes crumbly and evenly coated. Adding nuts at this stage enhances texture and flavor.
Sprinkle the topping evenly over the apples, making sure to cover all the fruit. This helps the crisp bake with a golden, crunchy top while the apples remain tender underneath.
Bake in the preheated oven for 35 to 40 minutes, or until the topping is golden and the apple filling is bubbly. Let it cool for at least 10 minutes before serving to allow the juices to settle. Serve warm, optionally with vanilla ice cream or whipped cream for extra indulgence.