Preheat the oven to 350°F (175°C). Lightly butter a 9x13-inch baking dish. Peel, core, and slice 6 Granny Smith apples into 1/4-inch-thick slices and place them in a large mixing bowl.

Make the apple filling

To the bowl with the sliced apples add 1 cup granulated sugar, 2 tablespoons fresh lemon juice, 2 tablespoons cornstarch dissolved in 2 tablespoons water, 1 teaspoon vanilla extract, 1/2 teaspoon ground nutmeg, and 1 teaspoon ground cinnamon. Toss thoroughly so each apple slice is evenly coated. Let sit 5 minutes to macerate and thicken slightly.

Transfer filling

Pour the apple mixture into the prepared 9x13-inch baking dish, spreading evenly into a single layer. Pour any accumulated juices from the bowl over the apples.

Prepare the oat topping

In a medium mixing bowl combine 1 1/2 cups rolled oats, 1 cup all-purpose flour, 1 cup packed brown sugar, 3/4 cup cold unsalted butter (cut into small cubes), 1 teaspoon ground cinnamon, and 1/4 teaspoon kosher salt. Using a pastry cutter, two forks, or your fingertips, cut the butter into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized butter bits remaining. This will create a crunchy, tender topping.

Assemble and bake

Evenly sprinkle the oat topping over the apple layer, pressing lightly so it adheres but still remains crumbly. Place the dish on a rimmed baking sheet in case of bubbling over. Bake in the preheated oven for 40–45 minutes, until the topping is golden brown and the apple filling is bubbling around the edges. If the topping is browning too quickly, loosely tent with foil for the last 10 minutes.

Rest and serve

Remove the crisp from the oven and allow it to rest 15 minutes — this helps the filling set. Serve warm in bowls or plates with scoops of vanilla ice cream or a dollop of whipped cream if desired.

Storage

Cool completely, cover tightly, and refrigerate for up to 4 days. Reheat individual portions in the microwave or warm a covered dish in a 325°F (160°C) oven until heated through. For best texture, reheat briefly to avoid soggy topping.