Preheat the oven to 375 degrees. Sieve the flour into a mixing bowl. Add the rock salt, egg whites and half the water. Mix. Gradually add the remaining water until a stiff dough forms. Roll the dough out until it is large enough to to wrap the fish. Saute the spinach leaves with garlic and olive oil. Season to taste. Cool in the refrigerator for a few minutes. Debone the fish without removing the head or tail keep the skin on. Fill the fish with spinach. Wrap the fish in the rock salt dough. Shape the dough in the shape of a fish. Bake for 25 minutes. Prepare the sauce: Briefly saute the shallots in a saucepan in one third of the olive oil. Add the garlic and white wine. Cook until reduced by half. Add all the other ingredients. Mix in the remaining olive oil. Season to taste. At the table, cut the crust lengthwise, removing only the top part of the crust (take care not to drop salt crystals on the fish). Remove the skin from the fish. Serve the fish on individual plates, placing the spinach on top and the sauce around the fish.