Preheat oven to 350°F (175°C). Grease a 9x13-inch baking dish.
In a large bowl, toss sliced apples with lemon juice, sugars, flour, cinnamon, and nutmeg. Spread evenly into the baking dish.
In another bowl, combine flour, oats, brown sugar, cinnamon, and salt. Cut in cold butter until mixture resembles coarse crumbs. Stir in nuts if using.
Sprinkle topping evenly over apples.
Bake for 40–45 minutes, until topping is golden brown and filling is bubbly.
Let sit for 10 minutes before serving. Enjoy warm with ice cream or whipped cream.