Usually, gremolata is made from parsley, garlic and lemon zest. Adding chopped almonds and olives to the basic recipe makes it more a brawny relish than a condiment.
Preheat oven to 500 degrees. Zest orange. Using a small, sharp knife, cut off top and bottom of orange, then stand orange up on cutting board and cut off the pith, following curve of fruit. Chop orange into 1/2-inch chunks and set aside.
Season fish with salt and pepper and place in a roasting dish. Mash together the butter and 1/2 teaspoon orange zest. Spread on fish. Roast until opaque and just cooked through, about 10 minutes.
Meanwhile, toss together orange chunks, parsley, olives, almonds, garlic, salt and pepper to taste and 1/2 teaspoon remaining orange zest. Taste and add more salt, pepper and zest if desired. Serve on top of fish.