Halve the beef heart, cut out all blood vessels and fat, then rinse under cold water.
Bring 2 liters of water to a boil in a large pot with salt, pepper, and the bay leaf. Add the halved beef heart. Clean the soup greens, cut them roughly, and add to the pot. Cook the heart over medium heat for 50 minutes.
Remove the heart with a slotted spoon and keep it warm in a bowl. Strain the cooking stock through a sieve and set aside.
Peel the small onion and chop finely. Heat 3 tablespoons of oil in a large pan over medium-high heat. Add the chopped onion and fry for 2–3 minutes until light yellow.
Sprinkle 35 g flour over the onion and stir constantly for 1 minute. Pour in 500 ml of the reserved cooking stock and bring to a boil, stirring frequently. Stir in 1 tablespoon tomato paste and 125 ml red wine. Season with salt and pepper to taste.
Finely dice the 2 pickled cucumbers and add them to the sauce along with the drained capers.
Cut the cooked beef heart into cubes about 2 cm in size. Add to the sauce and let simmer for 10 minutes. Serve hot.