Hi JJ, sorry for the delay, I was eating (LOL).
I usually get lamb's liver. As for cooking, I just google a recipe and follow it - either roast or fried which depends to a certain extent to how it is cut. You really need thin evenly-cut slices for frying.
Tonight I roasted it in halogen oven @ 200 for 30 minutes .
Beware of very thick cuts, can be like old boots when fully cooked (I think this is why so many people of my generation dislike liver, based on the great chunks which came with School Dinners in the Fifites and Sixties - I'm told it's better nowadays).