Step 1: Prepare the Apples : Preheat oven to 350°F (175°C). Peel, core, and thinly slice the apples. Place them in a large mixing bowl. Add lemon juice to prevent browning and toss to coat. Sprinkle granulated sugar, flour, cinnamon, nutmeg, and a pinch of salt over the apples. Toss until evenly coated.
Step 2: Make the Crisp Topping : In another bowl, combine oats, flour, brown sugar, cinnamon, and salt. Add the cold, cubed butter. Using a pastry cutter, fork, or your fingers, work the butter into the mixture until it resembles coarse crumbs with some pea-sized pieces. Stir in chopped nuts if using.
Step 3: Assemble the Apple Crisp : Grease a 9x13-inch baking dish (or similar size). Spread the apple mixture evenly on the bottom. Sprinkle the oat topping evenly over the apples, covering them fully.
Step 4: Bake : Bake in the preheated oven for 45–50 minutes, or until the topping is golden brown and the filling is bubbling. If the topping browns too quickly, tent with foil during the last 10 minutes.
Step 5: Cool and Serve : Remove the crisp from the oven and let it rest for at least 10–15 minutes before serving. This allows the juices to thicken slightly.