Preheat your oven to 375°F (190°C). Place the rack in the center of the oven.
Prepare your baking dish by buttering or spraying a 9-inch deep-dish pie plate or an 8x8-inch baking dish.
Make the topping: In a food processor, combine the flour, sugars, cinnamon, nutmeg, and salt. Add the cold butter and process until the mixture is crumbly with no large pieces of butter left. Stir in the oats and nuts, if using, and set aside.
Prepare the filling: In a large bowl, toss the apple chunks with the lemon juice, lemon zest, and white sugar until evenly coated.
Assemble the dish: Transfer the apple mixture into your prepared baking dish and spread the topping evenly over the apples.
Bake for about 30-40 minutes, or until the filling is bubbly and the topping is golden brown.
Cool on a wire rack for about 30 minutes before serving warm.