Using a sharp chef’s knife, cut the heart into 1-inch cubes. If they have a thick fat layer, you can trim some of it. Place the cubes 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 heart to a plate, and strain the cooking liquids into a saucepan. Return the meat to the slow cooker, cover it, and set it to warm.
Mix the cornstarch with 2 teaspoons of cold water. Stir this cornstarch slurry and 1 tablespoon of unsalted butter into the liquids in the saucepan. Cook over medium heat, whisking often, until the sauce thickens, about 2 minutes. Remove from the heat as soon as the sauce thickens.
Transfer the beef heart to plates. Top with the sauce, garnish with parsley and serve.
Video
Notes
The heart is a tough muscle. To make it edible, you should either slow cook it as I do here or slice it into "steaks" and flash fry them.
Some recipes instruct you to devein the heart, which is helpful when you flash-cook it. But slow cooking softens the tough parts, so trimming them is unnecessary.
You can thicken the cooking liquid by adding a tablespoon of cornstarch mixed with 2 tablespoons of cold water directly to the slow cooker pan at the end of cooking. Mix in this slurry, cover, and cook on HIGH for 30 minutes.
Keep the leftovers in the fridge, in an airtight container, for 3-4 days. Reheat them gently in the microwave, covered, at 50% power.