Heat the oven to 375°F and butter a 9-by-13-inch baking dish.
Toss the apples with the first brown sugar, 2 tablespoons flour, lemon juice, cinnamon and 1/4 teaspoon salt.
Spread the apples evenly in the dish.
Combine the oats, remaining flour, remaining brown sugar, walnuts and 1/2 teaspoon salt, then stir in the melted butter.
Crumble the topping over the apples in an even loose layer.
Bake for 45 to 50 minutes until the apples are tender, the center bubbles and the nuts are toasted; rest 20 minutes.