Preheat the Oven
Preheat your oven to 350°F (175°C). This moderate temperature allows the apples to cook through while the topping gets golden and crisp without burning.
Prepare the Crisp Topping
In a medium bowl, combine the oats, flour, cinnamon, brown sugar, salt, and cold butter. Use your hands to mix the ingredients, rubbing the dry ingredients into the butter until the mixture becomes crumbly with pea-sized pieces. The cold butter is essential for creating that perfect crisp texture. Set aside while you prepare the filling.
Make the Apple Filling
In a large bowl, combine the diced apples, lemon juice, cinnamon, sugar, brown sugar, and cornstarch. Stir thoroughly until all apple pieces are evenly coated with the spice and sugar mixture. The lemon juice prevents browning while the cornstarch will help thicken the juices as the apples bake.
Assemble the Crisp
Transfer the apple mixture to an 8×8 inch baking dish, spreading the apples in an even layer. Pour the melted butter evenly over the apples to add richness and help create a luscious sauce.
Add the Topping and Bake
Sprinkle the prepared oat topping evenly over the apple filling. Bake for 45-50 minutes, or until you notice the filling bubbling around the edges and the topping has turned a beautiful golden brown.
Serve
Remove from the oven and allow to cool slightly before serving. For an extra special treat, serve warm with a scoop of vanilla ice cream or a dollop of whipped cream.