Prepare and clean the beef heart by butterfly it open and trimming off any membranes, connective tissue, and blood vessels. Gently pound to flatten if needed. Rinse and pat dry before stuffing. (Refer to blog post for complete instructions and options).
Make the filling. Finely chop onion, apple, garlic cloves, and fresh herbs of your choice. Chop the bacon and spinach, grate the Parmesan cheese, and roughly chop the walnuts. Preheat oven to 325°F (165°C).
In a cast iron or stainless steel skillet over medium-high heat, cook the chopped bacon until just cooked but not crispy. Remove the bacon with a slotted spoon and set aside, leaving the rendered fat in the pan.
Add the chopped onion, garlic, and apple to the bacon fat and sauté for a few minutes, until softened and fragrant.
Stir in the chopped spinach and cook until wilted, about 2–3 minutes. Add the cooked bacon back to the skillet, stir, and sauté for another minute.
Transfer the filling to a bowl. Stir in the chopped walnuts and herbs and grated Parmesan cheese. Taste and season with extra salt if needed. Let the mixture cool slightly before stuffing.
Get ready to stuff the cleaned heart. Lay 3–4 long pieces of butcher’s twine on a cutting board or clean surface. Place the cleaned beef heart on top, inner side up. Spoon the filling evenly across the heart.
Roll the heart carefully and tightly, tucking in any loose filling as you go.
Use the butcher’s twine to tie the heart securely in several places. Season the outside with salt and pepper.
Heat your cooking fat of choice (like tallow, ghee, or butter) in a Dutch oven or oven-safe roaster over medium-high heat. Brown the stuffed heart on all sides for a few minutes to seal in the flavor.
Add a few tablespoons of bone broth or water to the bottom of the pot. Cover and transfer to a preheated 325°F (165°C) oven. Roast for about 2 hours, or until tender.
You can cook it medium-rare or continue roasting for a more well-done texture. Insert a fork to check tenderness.
Remove the cooked heart from the oven and let it rest for 10 minutes. Cut and remove the twine, then slice into 1-inch thick rounds. Serve warm with your choice of sides as a hearty and elegant main dish. You can use the cooking liquid as a flavorful sauce to spoon over the slices or serve in a separate bowl. Reduce the liquid for a few minutes over medium-low heat if necessary.