Heat the oil in a large skillet over a medium-high heat and add the garlic and onion. Cook for 2 to 3 minutes, then add the chopped tomato and stir. Cook for 30 seconds and then add the turmeric, mustard seeds, cumin seeds, curry leaves, cardamom seeds, salt and pepper.
Add the potatoes and stir. Add the sour cream and water, stir and bring to a boil for 15 minutes, until the potatoes are cooked through. They are done when the sauce has thickened and the potatoes are tender.