Hi Chris
I love lamb's hearts too. My mother would not cook them in any other way but to just slice them, not too thinly, marinate them with turmeric powder, salt and pepper. Slice one onion, slice green chillies, up to you how hot chillies you want it to be, I would sliced 2. put about 3 tbsp of cooking oil, when it's hot fry in the lamb's hearts, stirring it occasionally, put in about 1 tsp of sweet soy sauce or dark soy sauce with half tsp of sugar, this is optional, 1 tbsp of water just to make a little sauce out of the juice, have a taste. Put in the sliced onions and chillies. Once they're all cook through, although I like my onions to be a little crunchy, take the pan off the heat. We usually eat them with boil rice.
You can add in other spices and marinate the lamb's hearts, for example put coriander seeds, fennel seeds in a mortar and pound/crush them and then rub them to the sliced lamb's hearts. You don't have to leave them too long. 110-15 minutes if you're in a hurry will do.
Zarina