Chow.com offers an easy and highly rated recipe for apple oatmeal crisp. It serves eight people and uses a combination of Granny Smith and Golden Delicious apples.
Peel and core 2 1/2 pounds of medium Granny Smith and Golden Delicious apples, and then cut them into 1/4-inch-thick slices. Position a rack in the middle of the oven, and preheat the oven to 350 degrees Fahrenheit. Lightly butter an 8-by-8-inch baking dish.
Place the apples, 2 to 3 tablespoons granulated sugar, 3/4 teaspoon of ground cinnamon and 1/8 teaspoon fine salt in a large bowl. Toss to coat the apples, and then place the mixture in the baking dish.
Reusing the bowl the apples were mixed in, combine 1/2 cup of packed light brown sugar, 1/2 cup of uncooked rolled oats, 1/3 cup of all-purpose flour and 1/8 teaspoon of fine salt, and mix well. Cut 4 tablespoons cold unsalted butter into small pieces. Using your fingertips, work the butter into the oat mixture until the mixture forms small clumps.
Sprinkle the oat-and-butter topping evenly over the apples in the dish, and bake for about 50 to 60 minutes or until the streusel is crispy and the apples are tender. Cool the dish on a wire rack for at least 30 minutes before serving.