Step 1Pat the fish dry. Check for any hidden bones and remove them. Cut away blood line and any other dark patches and discard. Cut the fish across the grain into slices ⅛- to ¼- inch thick. Cut the slices into ¼- to ¾-inch pieces. Place in a glass bowl, add the citrus juices and stir to mix. Cover and refrigerate, stirring occasionally, until the fish is completely opaque, 2 to 3 hours.
Step 2Cut the onion half in half again vertically and then cut each piece crosswise into paper-thin slices. Rinse well and soak in cold water to cover for 30 minutes. Drain well and pat dry.
Step 3Drain fish thoroughly and place in a clean bowl. Add the onion, chives, olive oil, sumac, Aleppo pepper and sel gris and toss well. Taste and adjust the seasoning as desired. Serve with pita crisps.