Boil the potatoes in a seperate pan.
Cut the beans very small. Heat in a big pan the oil and fry the onion until the onion is translucent. Add the garlic and spices and let them fry for a minute. Now add the remaining ingredients well (including the boiled potatoes and beans) and bring it to a boil.
Turn the heat to low and simmer for 40 minutes with the lid on the pan simmer until potatoes are tender.
Stir in the curry occasionally a bit and add a little extra water if it threatens to boil dry. If the sauce at the end of the cooking time is too wet, you can still just simmer without a lid so that the extra moisture evaporates.