In a large bowl, mix together the vinegar, oil, salt, and pepper and marinate the peppers in this mixture for at least 2 hours. While the beans are simmering, roast the peppers over an open flame, gently turning them so the skin is equally charred, and carefully ensuring not to overcook them. Remove from flame and cover with a damp towel for a few minutes. After this, the peppers are ready to be peeled. Once the skin has been removed, briefly dip them in water to ensure the peppers are clean and ready to be stuffed.
For the filling: place the soaked black beans in a large pot or pressure cooker with the garlic and cover with water. Cook for 1 hour until beans are soft and creamy.
Once the beans are cooked, puree them in a blender until they form a smooth paste. In a casserole dish, heat the oil or lard, add the chopped onions, and fry until golden brown, about 10 minutes. Next, add the mashed beans and fry them together with the onions. Season with salt and simmer over medium heat for 10 minutes.
Gently slice open the peppers, remove the seeds, and fill the cavity with the refried bean paste. Gently fold the pepper over and press gently to maintain its shape.
To make the avocado sauce, slice 2 avocados in half, remove the stone, and spoon the avocado flesh out. Place this in a blender and puree with the raw and cooked green tomatoes, onion, garlic, cilantro, and chiles. Add a cup of cream or yogurt and season with salt.
Place the peppers on a plate, pour over the guacamole sauce and garnish with chopped cilantro.