Cut the liver into 5cm by 2cm strips, removing any sinew as you go. Just before frying season the flour with the cumin, salt and pepper and dust the liver.
Heat the butter and oil in a frying pan over a medium heat. When the butter begins to foam add the liver and fry on one side for about a minute until sealed and slightly brown
Turn over and fry for another minute. The liver should be juicy and ever so slightly pink inside.