Remove all tendons and skins from the beef heart. Cut the remaining muscle meat into roughly 2 cm cubes. Peel the carrots and slice them. Peel the celeriac and cut into fine sticks (or use jarred celeriac if available). Dice the onions and finely chop the garlic.
Heat 2 tablespoons of vegetable oil in a large roasting pan over medium-high heat. Add the heart cubes and sear for about 3-4 minutes until browned on all sides. Season with salt and pepper, then remove and set aside.
Add the remaining 2 tablespoons of oil to the pan over medium heat. Cook the onions for about 5 minutes until translucent. Add the carrots, celeriac, and garlic, and cook for 2 minutes, stirring occasionally.
Stir in the paprika powder and cook for 1 minute, stirring constantly. Pour in half of the red wine to deglaze the pan, scraping up any browned bits. Return the seared heart cubes to the pan and stir well to combine.
Cover the pan and simmer over low heat for 1 hour and 30 minutes. Stir occasionally and gradually add the remaining red wine throughout the cooking time.
Stir in the tomato paste. Season with salt and pepper to taste. Sprinkle with fresh parsley and marjoram before serving.