Preheat oven to 400°F (200°C) and get a baking dish that is about 10 x 8” (25 x 20 cm)
Cut beef heart into cubes and process in a food processor till it resembles ground meat, put it aside till you need it.
Slice the zucchini thinly lengthwise with a vegetable peeler or a mandolin. Lay some paper towels out and place zucchini slices on them and sprinkle with some salt
Let the zucchini rest for 10 minutes and then pat with paper towels. In the meantime, work on the meat sauce.
Heat the lard in a large frying pan, over medium heat. Add onion and garlic, and sauté until translucent. Add the ground beef and beef heart as well as the herbs, salt, and pepper. Stir to combine and cook until lightly browned.
Add in the tomato paste and marinara and stir until all is combined. Lower the temperature to medium-low and leave it to simmer for about 5-10 minutes stirring occasionally.
Add the sour cream, half of the shredded cheese, and the garlic to a medium-sized saucepan. Bring to a boil, and once bubbling, lower the temperature to medium-low. Let the sauce simmer for about 5 minutes while stirring at the same time or until the sauce has thickened. Add some salt and pepper to taste.
To assemble the lasagna: start by spreading a 1/3 of the meat sauce on the bottom of the baking dish. Cover the sauce with some of the cheese sauce, then cover with some of the zucchini slices in a single layer. Repeat with the rest of the layers and sprinkle the remainder of the cheese on top.
Put lasagna in the oven and bake for about 18-20 minutes. Turn on broiler and broil until the top of the lasagna is brown and bubbling, should take 2-3 minutes. Let the lasagna rest for at least 10 minutes before serving to let it set. Serve the lasagna salad leaves or spinach drizzled with olive oil and balsamic vinegar.