Heat the oven to 350 degrees. Butter a 13-by-9-inch baking dish.

Place the apples in a large bowl and toss with the 1/2 cup/100 grams brown sugar, cinnamon and lemon juice. Spread the apples in the prepared baking dish in an even layer.

In the same bowl, combine the flour, 1 cup/200 grams of the brown sugar, oats, pecans and salt. Add the butter, and stir with a fork until the crumbs are evenly moistened.

Press the crumb mixture together to create clumps of different sizes, and sprinkle on top of the apples. Transfer to the oven, and bake until the apples are tender and the crumb topping is crisp and deep golden brown, about 50 minutes. Serve warm or at room temperature, with vanilla ice cream if desired.