Preheat oven to 350°F (175°C). Lightly butter a 9×13-inch baking dish (or 9×9 for a deeper, thicker crisp).
Make the apple filling: Toss sliced apples with granulated sugar, brown sugar, cinnamon, nutmeg, lemon juice, and flour until evenly coated. Spread in an even layer in the prepared baking dish.
Make the oat topping: In a medium bowl, stir together rolled oats, flour, brown sugar, cinnamon, and salt. Add the cold butter cubes. Work the butter into the mixture with your fingertips, pinching until fully incorporated and the mixture holds together when squeezed but still breaks apart. Stir in pecans if using.
Assemble: Scatter the topping loosely over the apples in an uneven layer. Do not press it down.
Bake for 45–50 minutes, until the topping is deep golden brown and the apple filling is bubbling visibly around the edges. If the topping browns too fast, tent loosely with foil.
Rest 10 minutes before serving. Serve warm with vanilla ice cream.