1.Preheat oven to 450°F. Line a baking pan with parchment paper or tin foil. Pat fish dry with paper towels and place on the prepared pan.
2.Zest half of orange into a small bowl, then squeeze in juice from that half (2-3 tablespoons) and juice from half of the lime (about 1 tablespoon).
3.Slice the remaining orange and lime and set aside.
4.In another small bowl, combine the sumac and turmeric.
5.Drizzle fish with the olive oil, and evenly sprinkle with pepper and salt. Arrange fish on baking pan. Sprinkle fish with citrus mixture and then with the sumac mixture, flipping to cover both sides.
6.Roast until flaky and cooked through, about 10 minutes.
7.Serve, with orange and lime slices as garnish.
2 large haddock fillets (feel free to use bass, tilapia or your preferred white fish)