Combine flour, pecans, oats, sugars, cinnamon, and salt in a medium bowl. Stir in melted butter until mixture is thoroughly moistened and crumbly. Set aside.
Peel, core, and slice the apples into 1/2-inch-thick wedges. Toss apples, sugar, and cinnamon together in a large bowl; set aside.
Bring cider to a 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 2 tablespoons butter in now empty skillet over medium heat. When foaming subsides, add apples and cook, stirring frequently, until they begin to soften, 12-14 minutes. (Do not fully cook apples.) Remove pan from heat and gently stir in cider mixture until apples are coated.
Sprinkle topping evenly over apples, breaking up any large chunks. Place skillet on baking sheet and bake until apples are tender and topping is a deep golden brown, 15-20 minutes. Cool on a wire rack until warm, at least 15 minutes. Serve warm with whipped cream or ice cream.
Notes
If you do not have an oven-safe skillet, prepare the recipe right up to before you sprinkle the topping on. Transfer the filling to a 9x13 baking dish, then add the topping as directed. You may need to bake for an additional 5 minutes.