Wash and dry the cod, and cut into pieces about 1½ in. square. Melt the butter in a saucepan, fry the cod slightly, then take out and put aside. Add the sliced onion, flour and curry powder to the butter in the saucepan and fry 15 min., stirring constantly to prevent the onion becoming too brown, then pour in the stock. Stir until boiling, then simmer gently for 20 min. Strain and return to the saucepan, add lemon juice and seasoning to taste, bring nearly to boiling-point, then put in the fish. Cover closely, and heat gently until the fish becomes thoroughly impregnated with the flavour of the sauce. An occasional stir must be given to prevent the fish sticking to the bottom of the saucepan.