Coat a medium-sized slow cooker lightly with non-stick cooking spray. In the bottom of the slow cooker, toss apples with lemon juice, sugar, and flour. Combine oats, flour, brown sugar, cinnamon, and salt in a bowl. Cut in the diced butter until it resembles coarse meal. Spread topping evenly over the apples. Hint: I like to place a kitchen towel or paper towel under the lid of the crockpot. It helps to absorb any extra liquid and brown the topping. Purely optional. Cook on LOW for 4 - 6 hours or HIGH for 3 hours. Test the apples to be sure they are fully cooked. Serve warm with a scoop ice cream.