FOR THE TOPPING: Adjust oven rack to middle position and heat oven to 450°F. Combine flour, pecans, oats, 1/2 c. brown sugar, 1/4 c. granulated sugar, 1/2 tsp. cinnamon, and salt in medium bowl. Stir in melted butter until mixture is thoroughly moistened and crumbly. Set aside while preparing fruit filling.

FOR THE FILLING: Peel, core, and chop apples into 1/2 inch wedges. Toss apples, 1/4 c. granulated sugar, and 1/4 tsp. cinnamon together in large bowl; set aside. Bring cider to simmer in 12-inch oven-safe skillet over medium heat; cook until reduced to 1/2 cup, about 5 minutes. Transfer reduced cider to bowl or liquid measuring cup; stir in lemon juice and set aside.

Heat butter in now-empty skillet over medium heat. When foaming subsides, add apple mixture and cook, stirring frequently, until apples begin to soften and become translucent, 12 to 14 minutes. (Do not fully cook apples.) Remove pan from heat and gently stir in cider mixture until apples are coated. Transfer apples to 9x13 baking dish.

Sprinkle topping evenly over fruit, breaking up any large chunks. Bake until fruit is tender and topping is deep golden brown, 15-20 minutes. Cool on wire rack until warm, at least 15 minutes, and serve.