Ingredients

For the filling:

• 2 pounds Granny Smith apples, peeled, cored, and cut into 1/4-inch thick slices
• 1 tablespoon lemon juice
• 1/2 cup granulated sugar
• 2 tablespoons cornstarch
• 1 teaspoon ground cinnamon

For the topping:

• 12 tablespoons (1 1/2 sticks) unsalted butter, plus more for the baking dish
• 1 1/2 cups old-fashioned rolled oats
• 1 cup all-purpose flour
• 1/2 cup packed light brown sugar
• 1/2 teaspoon kosher salt
• 1 teaspoon pure vanilla extract

1. Arrange a rack in the middle of the oven and heat to 350°F. Coat a 9-inch square glass baking dish with butter; set aside.
2. Cut 12 tablespoons (1 1/2 sticks unsalted butter) into a few pieces and melt in a small saucepan over medium heat. Set aside to cool. Peel, core, and cut 2 pounds Granny Smith apples into 1/4-inch thick slices. Squeeze the juice of 1/2 small lemon until you get 1 tablespoon.
3. Whisk 1/2 cup granulated sugar, 2 tablespoons cornstarch, and 1 teaspoon ground cinnamon together in a large bowl to break up any lumps. Add the apples and lemon juice and toss gently to coat. Transfer to the baking dish and spread into an even layer.
4. Place the 1 1/2 cups old-fashioned rolled oats, 1 cup all-purpose flour, 1/2 cup packed light brown sugar, and 1/2 teaspoon kosher salt in a medium or large bowl — it can be the bowl that was just used for the apples — and stir until combined. Drizzle the melted butter and 1 teaspoon vanilla extract over the oat mixture and stir to combine.
5. Scatter the crisp topping evenly over the fruit mixture, leaving large clumps intact.
6. Bake until the fruit juices are bubbling around the edges of the baking dish and the topping is golden and firm to the touch, 50 to 60 minutes.
7. Let the crisp cool on a wire rack for at least 15 minutes before serving. If transporting to a picnic or party, let the crisp cool completely to give the fruit filling time to set.