Heat the oil in a stockpot over medium heat and cook the onions for about 10 minutes, until they start to brown. Add the turmeric and corn. Pierce the limes with a knife or fork and add them to the pot along with their soaking water. Add the stock and bring to a boil. Cover and simmer for 15 minutes, until the corn is just tender.
Squeeze the limes against the side of the pot with a long spoon to extract their concentrated flavor before removing them from the soup. Blend half of the soup in a blender, then return it to the pot. Add the saffron and season, to taste, with salt and pepper. Add lemon juice, to taste, and serve.