Cut the heart into 1-inch cubes using a sharp knife. Trim the harder pieces from the heart cavity off and, if desired, trim some of the outer fat layer off as well (but leave some tasty fat!).
Turn on the Instant Pot in "sauté" mode and add the butter and garlic (if desired). Once the pot is hot and the butter melts, add the heart meat. Season with salt and your choice of herbs and let the meat sauté for a couple of minutes, stirring frequently with a wooden spatula.
Add the bone broth or water and stir to combine. Turn the "sauté" mode off and set the Instant Pot on "slow cook" (normal sertting) for 3 hours.
Once the heart has slow-cooked for 3 hours, you can add your choice of vegetables (such as sweet potatoes, carrots, or potatoes). To do so, turn the Instant Pot off, open the lid, combine the cubed vegetables in the pot with the meat, and turn on the Instant Pot on "pressure cook" for as long as the vegetables require (about 15 minutes should be enough for most). If you want to serve the stew without any vegetables, skip this step.
Once the meat and vegetables (optional) are cooked, open the Instant Pot and turn it on "sauté" mode for about 10 minutes or until the stew sauce has thickened enough. Stir frequently with a wooden spatula.
Serve the stew warm and garnish with chopped fresh parsley, if desired.