For the Soup/Ash: To make the soup, place the oil in a large heavy pot over medium-high heat, toss in the onion, cassia, and turmeric, and cook until the onion is translucent, 4 to 5 minutes.
Add the rice, drained split peas, and 8 cups water, raise the heat, and bring to a boil, then lower the heat to maintain a strong simmer and cook for 1 to 1-1/2 hours, or until the split peas are tender.
While the soup is cooking, make the meatball mixture: Mix the onion thoroughly with the lamb. Mix in the salt and pepper.Set aside, covered, in the refrigerator.
Add the scallions, parsley, coriander, and mint to the soup and simmer for 30 minutes. Add another cup or two of water to thin it, as you wish, and bring back to a strong simmer.
Add the salt and 4 tablespoons pomegranate molasses and stir. Taste and add a little more pomegranate molasses if you like.
Make the meatballs about 15 minutes before you want to serve: Scoop up about a heaped teaspoon of the meat mixture for each and roll it into a ball between your wet palms, then drop it into the soup.
Let the soup continue to simmer while you make the toppings.
For the Toppings: Pour 2 tablespoons of the oil into a small skillet and heat over medium-high heat. Toss in the dried mint and immediately remove from the heat; it will fizz up a little. Set aside in a small bowl.
Heat the remaining 2 tablespoons oil in the skillet over high or medium-high heat, add the sliced onion, and fry until starting to brown and crisp, about 6 minutes. Set aside on a plate.
Ladle the hot soup into individual bowls, making sure to serve several meatballs in each, and top with a drizzle of mint oil, and with a sprinkling of fried onions if you wish.
NOTE: Short-grain rice or broken rice will break down more easily than long-grain rice as it cooks, and the starch from the rice helps thicken the soup.