1. Preheat oven to 375°.
2. Place pistachios in a food processor; process until finely ground.
3. Combine apple, turbinado sugar, cornstarch, and cardamom in a large bowl; toss well.
4. Stack 2 phyllo sheets on a large cutting board or work surface (cover remaining dough to keep from drying). Lightly brush phyllo stack with butter; sprinkle with about 1 tablespoon ground pistachios. Top with 2 more phyllo sheets. Spoon about 1/2 cup apple mixture along 1 short edge of phyllo stack, leaving a 2-inch border on all sides. Fold over the long edges of phyllo to cover about 1 1/2 inches of apple mixture on each end. Starting at short edge with 2-inch border, roll up jelly-roll fashion. (Do not roll tightly, or the strudel may split.) Place strudel, seam side down, on a baking sheet lined with parchment paper. Repeat procedure with remaining phyllo sheets, butter, pistachios, and apple mixture to form 8 strudels total.
5. Pierce each strudel 2 times with a fork. Brush tops with remaining butter. Bake at 375° for 35 minutes or until crisp and golden brown. Sprinkle evenly with 2 tablespoons powdered sugar. Serve strudels warm or at room temperature.