For fish?First remove some of the bovine pericardial chordae tendineae as that can be a big source of toxins and tickles the back of the throat. Otherwise I would season with garlic butter and cut into cubes. When the cubes are uniformly brown and start to soften you should be ready to serve. Bon Apetit!
What size fish? I find that with smaller fish, you can freeze a sizable piece of the heart and then, when you want it, run a cheese grater over it. That will provide a nice bunch of slivers that will be edible for smaller fish.