Long cooking times are sometimes recommended for heart to tenderize them, but it's not necessary. Cut them into small pieces as you would for a stir fry and you're golden. I made a curry with beef heart and the slices were probably 1/2" thick and 2" long, that worked very well. Some people recommend soaking livers a few hours to overnight in milk, not really sure why. The one time I really had livers I enjoyed they were chicken livers that just went straight into the pan. Make sure you don't overcook them and you should be fine.