Preheat the oven to 350 degrees F (175 degrees C).
In a medium bowl, combine the Unsalted Butter (1 stick), All-Purpose Flour (3/4 cup), Brown Sugar (1/2 cup), Old Fashioned Rolled Oats (1/2 cup), Walnut (1/2 cup) and Kosher Salt (3/4 teaspoon). Using a wooden spoon (or clean hands), mix until well combined. Gently fold in the Shredded Sharp Cheddar (1 cup), then set aside.
Peel, core, and thinly slice all the Granny Smith Apple (1.5 pound) and Fuji Apple (1.5 pound) and place them in a large bowl.
In a small bowl, whisk together the Lemon Juice (3 tablespoon) and the remaining All-Purpose Flour (1 tablespoon) until completely smooth. Add this mixture to the apples, along with Ground Cinnamon (1 teaspoon) and the remaining Brown Sugar (1/4 cup). Toss well.
Transfer the apples to a 9×9-inch square baking dish and spread into an even layer. Crumble the topping mixture evenly on top, covering the entire surface of the apples.
Bake for 40-50 minutes until bubbling and golden brown on top.
Cool for 10 minutes, then serve with scoops of Vanilla Ice Cream (to taste).