Step 1

Heat the oven to 350°F and prepare the baking dish. 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.

Step 2

Make the fruit filling. Whisk the sugar and cornstarch 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.

Step 3

Make the crisp topping. Place the oats, flour, brown sugar, and 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 butter and vanilla over the oat mixture and stir to combine.

Step 4

Top the crisp. Scatter the crisp topping evenly over the fruit mixture, leaving large clumps intact.

Step 5

Bake the crisp for 50 to 60 minutes. 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.

Step 6

Cool the crisp. 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.