Get Ready to Bake: Start by preheating your oven to 375°F. Butter a 9x13-inch casserole dish thoroughly and set it aside for later use.
Prepare the Crumb Topping: In a large mixing bowl, combine the flour, oats, brown sugar, granulated sugar, cinnamon, and salt. Add the diced butter to the bowl and mix everything together using your hands until the mixture forms small clumps about the size of peas. Stir in the sliced or slivered almonds, then set the mixture aside.
Slice and Prep the Apples: Peel, core, and cut the apples into thin slices, roughly ¼ inch thick, ensuring you have about 12 cups of sliced apples. Place the apples and cranberries in a large mixing bowl. Drizzle the lemon juice over the fruit and gently toss to evenly coat.
Mix the Filling Ingredients: In another bowl, stir together the granulated sugar, flour, and cinnamon. Sprinkle this dry mixture evenly over the apples and cranberries. Toss everything lightly to ensure the fruit is coated. Once mixed, pour the fruit filling into the prepared casserole dish, spreading it out evenly.
Add the Topping and Bake: Sprinkle the crumb topping evenly over the fruit mixture in the casserole dish. Place the dish in the oven and bake for 45 to 50 minutes, or until the topping is a rich golden brown and the fruit filling is bubbling around the edges. The center of the crisp should reach an internal temperature of 175°F.
Cool and Serve: Allow the crisp to cool slightly before serving. For a decadent touch, add a scoop of vanilla ice cream on top and enjoy.