Rinse the heart under cold running water and pat it really dry.
Remove any visible strings, arteries and blood vessels that may have been left behind; trim off excess fat and place the heart to marinate overnight in balsamic vinegar.
Remove the heart from the marinade, pat it really dry once again, and cut the meat into thin strips; sprinkle generously with salt and pepper and set aside.
Cook bacon in a large skillet set over medium heat until crispy. Remove to a plate but leave the fat in pan.
Add the onions to the pan with the bacon fat and cook until soft and caramelized, about 5 minutes. Add mushrooms and continue cooking until softened and golden. Remove this to a plate, next to the bacon.
Return the pan to the heat source, add more fat to the pan if necessary, then add the meat to the pan in a single layer, making sure that the strips do not touch, and cook until brown on all sides. Work in several batches if you have to so you don’t overcrowd the pan. Remove the cooked pieces of meat to the plate with the mushrooms, onions and bacon.
When all the meat is cooked, add it back to the skillet, along with the cooking juices, onions, mushrooms and bacon.
Mix the bone broth, Dijon mustard, balsamic vinegar and rosemary together in a large measuring cup and pour that in the pan.
Bring to the boil then lower the heat and simmer slowly, uncovered, for about 30 minutes or until the broth has reduced by about half and the beef heart is somewhat tender.
Dilute the arrowroot powder in 2 tablespoons of water, add that to the simmering sauce and stir well with a spoon until sauce is thickened, about one or two minutes.
Serve over spiralized zucchini and garnish with fresh parsley.