In a large pot, add the 4 cups of chicken or vegetable stock as well as the coconut milk. Add a sprinkle of salt. Bring to a low boil and then add the 4-5 ears of corn. Cover and simmer for about 10 minutes, turning to the ears of corn as necessary to ensure even cooking.
After 10 minutes, turn off the heat and set the corn and coconut milk aside
Meanwhile, using an additional pot (I like to use a dutch oven for my soups), heat 3 tbs of olive oil over medium heat, and add the onions and potatoes. Cook for 3-5 minutes, stirring occasionally to ensure that the potatoes don't stick to the bottom of the pot
Once the onions are translucent, and the potatoes begin to brown, add ½ tsp of salt, the curry paste, ginger and lemon grass. Stir to combine. Allow the spices to cook for 1 minute (careful not to burn the curry paste).
Meanwhile, remove the ears of corn from the coconut/chicken stock mixture. Set aside to cool.
Then pour the remaining coconut/stock mixture directly into the pot with the potatoes/onion. Carefully stir everything together, and bring to a low simmer over medium/low heat. Cook for an additional 15-20 minutes (until potatoes are just tender. Don't overcook potatoes).
Carefully, using a sharp chef's knife, remove corn kernels from the cobs and add back into the soup.
Allow the soup to come together and cook for an additional minute or two.
To finish, check seasoning, and add additional salt/pepper as necessary. Finish with a handful of chopped cilantro and the juice of two limes.