Prepare the stuffed flap steak: In a bowl, add the rice and the flap steak trimmings and season generously with salt and pepper. Place the flap steak on your work surface with the grain running horizontally. Season generously with salt and pepper on both sides. Spread the rice mixture over the bottom half of the steak, leaving a 1½-inch border. Fold the top half over the rice and sew the edges closed with small stitches about ½-inch apart.

Prepare the kuklas (optional): Heat 2 tablespoons of neutral oil in a pan over medium heat, add the chopped onion and fry until golden, about 5-7 minutes. Set aside to cool.

Soak the challah pieces in a bowl with water for 3-5 minutes, until soft. Then squeeze out all the liquid.

In a bowl, mix together fried onions, squeezed challah, salt, and spices until a uniform mixture forms. If the mixture feels too dry, add the egg. If it feels too wet, add some breadcrumbs until the mixture is easy to work with. Then shape the mixture into 20-24 round and flat dumplings, about 2 inches wide. 

Heat 1½ inches oil in a deep skillet over high heat until the surface shimmers, but does not smoke. To check if the oil is hot enough, insert the end of a wooden spoon into the skillet — if it bubbles gently around the wood, the oil is ready. If it bubbles vigorously, it's too hot. Fry in batches on both sides until golden brown, about 3 minutes per side and transfer to a tray lined with paper towels. Set aside.

Season the marrow bones and short rib generously with salt and pepper. Set an 8-quart, heavy-bottomed, oven-safe pot over high heat. Add 2 tablespoons of oil and sear the marrow bones and meat until well browned, about 3–5 minutes per side. Leave the meat drippings in the pot. 

Preheat the oven to 250°F.

In the same pot, layer the ingredients in the following order: bones first, then beans, garlic, ¼ cup oil, and short rib. Then, place the stuffed meat on top.

In a bowl, mix the spices with 3 cups of water and pour into the pot. Add more water if needed until everything is covered. Bring to a boil over high heat. Lower the heat to medium, cover with a lid, and simmer for 30 minutes, turning the stuffed meat halfway so it absorbs the liquid evenly. 

After 30 minutes, use big tongs to carefully lift the stuffed meat and transfer to a plate. Add the potatoes, sweet potatoes, and hard-boiled eggs. If making kuklas, add them now as well. Leave a space in the center of the pot for the stuffed meat. Place the stuffed meat back in the pot and add more water if necessary — it should nearly cover the stuffed meat.

Cover the pot with aluminum foil, then seal tightly with a lid and place in the oven for 14-16 hours. After 4 hours, check the liquid and add boiling water if needed. Repeat this once or twice during cooking. By the end, most of the water should be absorbed into the hamin — add just enough to keep it from burning.

For serving, transfer the stuffed meat to a cutting board. Using kitchen shears, remove the twine, then slice against the grain and serve hot with the hamin.