Preheat oven to 350F and grease an 8" square baking dish.
Combine sliced apples with lemon juice, sugar, and cinnamon. Spread evenly in bottom of dish and dot with 1 Tbsp butter.
In a medium bowl, combine flour, brown sugar, and oats. Cut in butter until the mixture resembles coarse meal.***
Sprinkle mixture evenly over apples. Bake for 35-40mins until apples are tender.
Serve warm or cold with whipped cream or ice cream as desired.
* I used a mixture of Granny Smith and Honey Crisp. ** 1/2 cup makes this recipe pretty sweet, so reduce to 1/4 cup if you'd like to tone it down. *** I use a pastry blender for this, but you could use 2 butter knives or even your hands to beak up the butter.