Wash and clean trotters well, remove any hair. In a large saucepan (that can hold 5 pounds of cooked rice), add trotters, 2 whole garlic bulbs, and enough water to cover them. Boil on medium heat for about 5 to 6 hours. During this process if you see that water is not enough and trotters are not yet absolutely tender, add in some more boiling water (do not put cold water.)
When trotters become tender and about 4 or 5 cups of soup is left, remove from heat. Discard all big bones; before discarding remove all soft geletine and bone marrow. Set this and small bones aside, strain all of the soup through a fine sieve.
Put banaspati or oil in the same pan and fry onions until brown on medium heat. Remove onions only from oil and grind them. Mix onion paste, plain yogurt, salt, and all dry spices, put them into the saucepan containing the banaspati.
Keep stirring until all water of the yogurt dries up; add trotters and soup. Place the pan on a heavy griddle or tava and put on low hear for atleast one hour.
When soup thickens, remove from heat. Add lemon juice. Serve while it is hot, garnish with coriander leaves and green chillies.