Make the stock and keep it warm over low heat.
Heat the olive oil in a soup pot and add the onion and 1/2 tsp. salt. Saute over medium heat until it begins to release its juices, about 5 minutes, then add the garlic, cumin, coriander, ginger, and a few pinches of cayenne. Cook until the onion is very soft, about 10 minutes, adding a little stock if it sticks to the pan.
Add the carrots, potato, 1 tsp. salt, and 1 qt. stock. Bring to a gentle boil, then reduce the heat, cover, and simmer until the carrots are very tender, about 15 minutes. Puree the soup in a blender or food processor until smooth, using a little extra stock if needed. Return to the pot, add the orange juice, and thin with stock to the desired consistency. Season with salt to taste and, for additional heat, a pinch or two of cayenne. Garnish each serving with a swirl of creme fraiche and sprinkle with cilantro.
Makes 9 to 10 cups.
NOTE:" Cumin and coriander are the smooth background flavors, but it's the fresh ginger, the orange juice, and a hint of cayenne that make this soup sparkle. A little potato adds silky texture; if your carrots aren't sweet, use a sweet potato instead."