Rinse the heart well under cold running water and pat it really dry.
Cut the heart open (if your butcher hasn’t done that already) and remove any visible strings, arteries and blood vessels that may have been left behind; trim off excess fat, if any. Set aside.
In a large skillet set over medium heat, cook the bacon until nice and crispy then remove it to a plate but leave the fat in pan.
Add the onion, mushrooms salt and pepper to the pan and cook until softened and fragrant, about 4-5 minutes. Stir in the cinnamon and nutmeg, then throw in the spinach, crispy bacon and chopped garlic. Stir well until the spinach is completely wilted and continue cooking until all the liquid at the bottom of the pan has evaporated. Let cool for a few minutes.
Preheat your oven to 275°F
Lay the beef heart fatty side down on a work surface; spoon the stuffing over it and spread it all the way to the edge. Roll the beef heart and tie it up with a few strands of butcher's twine. You might want to run one piece across lengthwise and 2 to 3 pieces crosswise. (if you end up with a little too much stuffing, hang on to it: you can reheat it later in a small skillet and serve it alongside the roast)
Melt a fair amount of cooking fat in a large oven-safe skillet set over scorching high heat. Sear the beef heart for 2 to 3 minutes per side, or until a nice golden crust forms. When the meat is nicely seared all around, transfer the skillet to the oven and cook the beef heart uncovered for 15 to 20 minutes per pound, depending on desired doneness (rare to medium-rare)
Remove the roast from the oven, tent it loosely with aluminum foil and let it rest for at least 10 minutes before slicing and serving.