Heat oil in a large pot and add panch phoran and green chilli. Cook for a couple of minutes until cumin seeds begin to lightly brown.
Add the whole peeled potatoes and the rest of the ingredients, stir and bring to a simmer. Cover and cook for 15 minutes.
Turn potatoes over, stir gently then replace the lid and cook for a further 15 min or until potatoes are cooked through.
Reduce heat, remove the bay leaf and smash the potatoes with a spatula or fork mashing some to thicken the soup.
Serve as is or with steamed rice.