Preheat oven to 375 degrees F (190 degrees C). Butter a 9x13-inch baking dish.
Spread apple slices into prepared baking pan. Dot with 1 tablespoon butter pieces and sprinkle with lemon juice. Mix flour, rolled oats, brown sugar, cinnamon, and salt together in a bowl. Cut butter into oat mixture until the mixture resembles coarse crumbs; spread over apples.
Bake in the preheated oven until topping is browned and apples are soft and tender, about 30 minutes.
