Preheat oven to 425 degrees. Line a rimmed baking sheet with aluminum foil. Place fish on sheet; season with salt and pepper, and set aside.
In a food processor, pulse pistachios, parsley, and garlic until coarsely chopped, 15 seconds. With machine running, add oil in a steady stream; blend until mixture is finely chopped. Season with salt and pepper.
Top fish with pistachio mixture. Roast until fish is opaque throughout, 12 to 14 minutes. Serve with lemon wedges.