Preheat oven to 350°F (175°C). Butter a 2-quart baking dish.
Prepare the Filling: In the prepared dish, combine apples, lemon juice, sugar, and flour. Toss to coat.
Make the Topping: In a bowl, combine flour, oats, brown sugar, cinnamon, and salt. Cut in butter (use a pastry cutter, food processor or your hands) until mixture resembles coarse crumbs.
Assemble: Sprinkle topping evenly over fruit filling.
Bake: Bake until fruit is bubbling and topping is golden brown, about 45 minutes.
Serve: Serve warm with vanilla bean ice cream or fresh whipped cream.