
Preheat the oven to 350°F. Line a rimmed baking sheet with foil. In a 9- or 10-inch cast-iron skillet, melt 2 tablespoons of the butter over medium heat. Remove from the heat. In a large bowl, toss together the apples, raspberries, granulated sugar, cornstarch, vanilla, and ¼ teaspoon salt. Scrape the apple mixture and juices into the skillet; spread into an even layer.
In the same bowl, combine the flour, oats, pistachios, brown sugar, cinnamon, and the remaining ¼ teaspoon salt. Add the remaining 1/2 cup of the butter and mash with your fingers until the butter is incorporated and crumbs form. Squeeze the mixture into clumps and sprinkle over the apples in the skillet.
Place the skillet on the prepared baking sheet. Bake until the crisp is golden and bubbling, 40 to 50 minutes. (Cover loosely with foil for the last 15 minutes if the top is getting too brown.) Let cool about 10 minutes. Serve with ice cream.