1. For the topping: Process 1/3 cup oats in food processor until finely ground, about 20 seconds. Transfer processed oats to medium bowl and stir in remaining topping ingredients.
2. For the filling: Adjust oven rack to upper-middle position and heat oven to 400 F. Cut half of apples into 1inch chunks. Slice remaining apples into 1/4inch thick wedges.
3. Bring apple chunks, juice, sugar, and cinnamon to simmer in 12-inch nonstick skillet over medium heat. Cover and cook, stirring occasionally, until apples are tender, about 15 minutes.
4. Transfer mixture to large bowl and mash smooth with potato masher. Measure out and reserve 1 tablespoon of mashed apple for topping. Return remaining mashed apple mixture to skillet and add sliced apples. Cover and cook over medium heat until slices begin to soften, 3 to 4 minutes. Off heat, stir in lemon juice.
5. Scrape filling into 8-inch square baking dish and smooth into even layer. Stir reserved mashed apple into topping mixture until topping is crumbly. Sprinkle topping over filling. Bake until juices are bubbling and topping is deep golden brown, about 25 minutes. Cool on wire rack until warm, about 15 minutes. Serves 6.