Roast the chiles over a flame until the skins bubble and are partially charred, then put them in a bowl , cover with a plate and set aside to steam (or buy chiles that are already roasted).
Toast the coriander in a dry skillet until fragrant and the color changes, then cool and then grind to a powder using a mortar and pestle or spice mill.
In a large soup pot over medium heat heat the oil and then add the onion, potatoes and coriander, turning to coat for 5-10 minutes or until potatoes are golden.
While potatoes are cooking slip skins off the chiles, discarding the seeds and chopping the chiles into large pieces.
Add chiles, 1 teaspoon salt and the water or vegetable broth into the soup pot. Bring to a boil and then lower heat to a simmer and cook covered for 25-30 minutes or until potatoes are soft. Season with salt and pepper.
In a small saucepan bring a small amount of water to a boil, submerge the cilantro for a few seconds, drain and rinse under cold water. Chop the cilantro and stir it into the sour cream.