Preheat your oven to 350°F (175°C). Lightly grease a 9×13-inch baking dish or a 10-inch cast-iron skillet with butter or cooking spray.
In a large bowl, toss the sliced apples with fresh lemon juice. Add the granulated sugar, flour, cinnamon, cardamom, nutmeg, and salt. Toss until the apples are evenly coated.
In a medium bowl, combine the rolled oats, flour, brown sugar, chopped pistachios (reserving 2 tablespoons for garnish), cardamom, cinnamon, and salt. Mix well to distribute the spices evenly.
Add the cold cubed butter to the dry topping mixture. Using your fingertips, a pastry cutter, or two forks, work the butter into the dry ingredients until the mixture resembles coarse crumbs. Drizzle with vanilla extract and toss lightly.
Transfer the apple mixture to your prepared baking dish, spreading it evenly. Sprinkle the pistachio topping over the apples, covering the entire surface. Place the baking dish on a larger rimmed baking sheet to catch any potential overflow.
Bake in the preheated oven for 40-45 minutes, or until the filling is bubbling at the edges and the topping is golden brown. If the topping begins to darken too quickly, loosely cover with aluminum foil.
Remove from the oven and let cool for 15-20 minutes before serving. Just before serving, sprinkle with the reserved chopped pistachios for extra texture and visual appeal.