Sole with flour

4 tablespoons butter , Divided 2 tablespoons olive oil 3 tablespoons flour 2 cups fish stock or 2 cups chicken stock 1 cup mushroom , sliced 1/2 cup celery ,[...]

plain flour (enough to coat the fish) 2 fillets of sole (about 300g, skinned and cleaned) 40 g unsalted butter 2 tablespoons sunflower oil fine sea salt 1/2[...]