How to Make It
Preheat the oven to 400° and position a rack in the lower third. In a large bowl, using a handheld electric mixer, beat the egg whites until soft peaks form. Add the salt and beat just to combine.
Line a large baking sheet with parchment paper. Spread a 1/2-inch-thick layer of the salt foam on the paper in the shape of the fish. Brush the fish lightly with oil and set it on the salt foam. Stuff the orange slices and thyme sprigs in the cavity and cover the fish with the remaining salt foam; be sure to coat it completely and seal any gaps. Poke a hole through the salt into the thickest part of the fish, just behind the head. Bake the fish for about 40 minutes, until the crust is deeply golden and an instant-read thermometer inserted in the center of the fish registers 140°. Let the fish rest for 10 minutes.
Meanwhile, in a large, deep skillet, melt 2 tablespoons of the butter. Add the fennel, cover and cook over moderate heat, stirring occasionally, until softened, about 5 minutes. Uncover and cook, stirring frequently, until the fennel is tender and browned, about 10 minutes longer. Add the wine and cook until evaporated. Add the olives and the orange zest and juice and cook for 1 minute. Stir in the chives and the remaining 1 tablespoon of butter.
Crack the salt crust and remove the top and sides. Carefully remove the skin from the flesh and, using an offset spatula, lift the fillets off the center bone. Transfer the fillets to a plate. Carefully lift up the bone and discard. Lift the flesh from the underside of the fish, leaving the skin and salt behind. Serve the fish with the fennel and olives.