For the topping: Adjust oven rack to middle position and heat oven to 450 degrees. Combine flour, pecans, oats, brown sugar, granulated sugar, cinnamon, and salt in medium bowl. Stir in melted butter until mixture resembles wet sand and no dry flour remains. Refrigerate until ready to use.

For the filling: In separate bowl, toss together apples, granulated sugar, and cinnamon, if using; set aside. Bring cider to simmer in 10-inch ovensafe skillet over medium heat and cook until reduced to 1/3 cup, about 3 minutes. Transfer reduced cider to small bowl and stir in lemon juice.

Melt butter in now-empty skillet over medium heat. Add apple mixture and cook, stirring frequently, until apples begin to soften and become translucent, 12 to 14 minutes. (Do not fully cook apples.) Off heat, gently stir in cider mixture until apples are coated.

Sprinkle topping evenly over fruit, breaking up any large pieces. Place skillet on baking sheet and bake until fruit is tender and topping is deep golden brown, 15 to 20 minutes. Let cool on wire rack for 15 minutes before serving.