Set aside cubed potatoes in cold water. Heat oil in a large pot and add cumin seeds, frying until they crackle. Reduce heat and add drained potatoes. Add enough fresh water to cover the potatoes and another 3 inches above. Add tumeric, salt and cayenne pepper to taste. Bring to a boil, then lower heat and cook potatoes until tender. Stir periodically and add more water if necessary. Mash some of the potatoes to make a thick soup-like consistency. When serving, add lemon juice and garnish of garam masala and coriander leaves.