Carve duck ready for serving. Chop onion finely, put into a saucepan, fry in butter, stir in flour and cook gently until brown. Add stock, stir until boiling, allow to simmer 10 min. Cut orange rind into very thin strips, add to the sauce with the orange juice, claret and pieces of duck. Season. Simmer very gently for ½ hr. Dish the meat and pour sauce over.