Preheat the oven to 140 °C. Roll the fish in coarse salt. After 8 minutes, rinse the fish under cold running water. Pat dry with a clean towel. Heat a fireproof pan with the butter and olive oil. Color the fish on the skin side. Season with pepper and a little salt. Turn the fish over, add butter and cook for another 5 minutes in the oven at 140 °C. Bake the fish in the oven with the melted butter. Remove the skin from the fish and season with some fleur de sel.