Fry the onions in 3-4 tablespoons of oil on very low heat until soft and golden, keeping the lid on and stirring occasionally to keep the onions from burning. It may take 30-45 minutes because of the large quantity.
Trim off skin and excess, but not all, fat from meat and cut into 1½-inch pieces. Fry in 3-4 tablespoons of oil in batches in a large heavy-bottomed pan, turning the pieces to brown them all over.
Add the garlic, cumin, and coriander to the onions and stir well for a minute or so, then add the spiced onions to the meat. Cover with about 4 cups of water and bring to a boil. Remove the scum and add cinnamon, cloves, and chiles, then simmer, covered, on low heat for 1½-2 hours (I find that it takes less time), or until meat is very tender.
Now add the well-washed potatoes and add about 2½ cups water (I don’t), just enough to cover. Add the coconut milk, tamarind, and sugar and simmer for 30–40 minutes until the potatoes are tender.