If the heart comes with a thick layer of fat, lightly trim it (but don't go nuts about it, you can leave a thin layer of fat).
Using a sharp chef’s knife, cut the heart into 1-inch cubes. Place them in the slow cooker pan.
Add the beef stock. Sprinkle the meat with salt, pepper, garlic powder, cumin, paprika, and oregano.
Cover and cook on HIGH for 4 hours.
Use a slotted spoon to temporarily transfer the meat to a plate, and strain the cooking liquids into a saucepan. Return the meat to the slow cooker, cover, and set to warm.
To the liquids in the saucepan, add 1 tablespoon unsalted butter and 1 teaspoon cornstarch mixed with 2 teaspoons of cold water. Cook over medium heat, whisking often, until the sauce thickens, about 2 minutes. Remove from heat as soon as the sauce thickens.*
Transfer the beef heart to plates. Top with the sauce, garnish with parsley and serve.
The heart is a tough muscle. To make it edible, you need to either slow cook it as I do here, or you can slice it into "steaks" and flash fry - no more than 30 seconds per side, keeping it rare. If you overcook it, it will be tough and inedible. Since the USDA recommends cooking organ meats to 160°F, this slow-cooked stew is a good solution if you don't want to eat it rare.Some recipes instruct you to devein the heart, and this might be useful when you flash-cook it. But I find that slow cooking takes care of any tough parts, so there’s no need to trim them.Trimming a thick layer of fat is sometimes easier if you first cube the heart, then use a knife to trim the fat off some of the cubes.*Another way to thicken the cooking liquid is to add a tablespoon of cornstarch mixed with 2 tablespoons of cold water directly to the slow cooker pan at the end of cooking. Mix this slurry in, cover, and cook on HIGH for 30 minutes.