Working with one ear of corn at a time, stand it upright, stem end down, in a large bowl. Using a sharp knife, cut downward along the cob, removing the kernels and rotating the cob a quarter turn after each cut. Reserve the cobs. Set the kernels aside. Cut one of the onions into thin slices and set aside. Cut the other onion into ½-inch dice and set aside.

In a heavy soup pot, heat the vegetable oil over medium-high heat and swirl to coat the pan. Add the sliced onions and sauté, stirring frequently, until soft and just beginning to brown at the edges, 10 minutes. Add the reserved corncobs and the water. Bring to a boil, reduce the heat to a simmer, partially cover the pot, and cook until reduced to 4 cups, about 30 minutes. Using tongs, remove the cobs from the pot and discard. Pour the corn broth through a fine-mesh sieve into a clean bowl or, preferably, a 4-cup glass measure. Press down on the solids to extract as much liquid as possible. You should have 4 cups of strained broth. Set aside. Clean the soup pot.

Return the soup pot to medium heat, add the olive oil, and swirl to coat the bottom of the pot. Add the diced onion, celery, poblano chile, and thyme. Sauté the vegetables until soft but not brown, 5 to 7 minutes. Add the reserved corn kernels, strained broth, cream, salt, and cayenne. Bring to a simmer and cook just until the corn is crisp-tender, 7 minutes. Add the salmon and cook at a bare simmer just until the salmon is cooked through, 3 minutes. Taste and adjust the seasoning. Divide among heated deep bowls, garnish with cilantro leaves, and serve immediately. Pass the lime wedges and squeeze a little lime juice into the soup.