Preheat and prepare: Heat the oven to 350°F (175°C). Grease your baking dish with butter or nonstick spray.
Prep the apples: Peel, core, and slice the apples into 1/4-inch thick slices. Aim for even pieces so they cook at the same rate.
Make the filling: In a large bowl, toss the apples with granulated sugar, 2 tablespoons flour, 1 teaspoon cinnamon, 1/4 teaspoon salt, lemon juice, vanilla (if using), and cornstarch (if using).
The mixture should look lightly coated.
Transfer to dish: Spread the apples into an even layer in the baking dish. Press down gently to remove big gaps.
Mix the topping: In another bowl, combine the remaining flour, brown sugar, oats, 1 teaspoon cinnamon, 1/4 teaspoon salt, and nutmeg (if using). Add the cold butter cubes and cut them in with a pastry cutter or your fingers until the mixture forms pea-sized crumbs with a few larger clumps.
Top the apples: Sprinkle the crumble evenly over the apples.
Don’t press it down too hard—you want a craggy surface for maximum crunch.
Bake: Place the dish on a baking sheet to catch any drips. Bake for 40–50 minutes, until the topping is golden brown and the apple juices are bubbling around the edges.
Rest: Let the crisp sit for at least 10–15 minutes. This helps the juices thicken and makes serving easier.
Serve: Enjoy warm as is, or with vanilla ice cream, whipped cream, or a drizzle of caramel sauce.