Warm the oil in a large pot over medium heat. Cook the onion with a pinch of salt for 6–7 minutes until soft.
Add the ginger, garlic and cumin and cook, stirring, for 1 minute.
Add the carrots, potato, broth and salt. Bring to a boil, then lower the heat, cover partially and simmer for 20–25 minutes until the carrots are very tender.
Blend until smooth with a stick blender, or in batches in a countertop blender with the lid vented and covered with a towel.
Stir in the citrus juice, taste and adjust the salt. Thin with a little broth or water if needed. Serve with a swirl of yogurt and herbs.