1 large yellow onion, quartered, peeled (root end left intact)
2 large fresh poblano chiles (8 oz.)
1½ lbs. frozen corn kernels, thawed and drained, divided
2 tsp. kosher salt
4 cups plus 3 tbsp. water, divided
2 tbsp. fine cornmeal or masa harina
Crumbled queso fresco and fresh cilantro, for serving
Divide the white and green parts of the scallions. Chop the green portion and set aside. Place a large cast iron skillet over medium-high heat. Add the white portion of the scallions, the onion and poblanos to the skillet. Let cook undisturbed, about 5 minutes, until charred on the bottom. Turn with tongs, until the second side is charred, about 5 minutes more. (Alternatively, the vegetables can be roasted at high heat or broiled in the oven until charred.)
Remove the scallions from the skillet to a blender or food processor. Continue cooking the onions and chiles until charred on all sides, turning as needed, 5-10 minutes more. Remove the pan from the heat. Trim away the root portion of the onion quarters and discard. Place the onions in the blender. Put the charred chiles in a medium bowl and cover with a plate or foil. Let stand for 15 minutes.
Remove the chiles from the bowl. The steam should have loosened the skins. Peel away the skins, trim and remove the seeds and stems. Cut into thin strips, and cut again into 1-inch lengths.
Add 2½ cups of corn, the salt and 2 cups of the water to the blender. Puree until smooth, about 1 minute. (If you prefer a chunkier soup, pulse until it reaches your preferred consistency.) Transfer the puree to the skillet and place over medium-high heat. Stir in the remaining corn and up to 2 cups of water (I added about 1½ cups). Bring to a boil. Reduce the heat to a simmer and let boil until slightly reduced, about 8 minutes.
In a small bowl, combine the 3 tablespoons of water and 2 tablespoons cornmeal or masa. Whisk until smooth. Blend into the soup in the skillet. Add in the chopped chiles. Simmer, stirring occasionally, until the soup has thickened somewhat, about 3-4 minutes. Remove from the heat. Serve topped with queso fresco and cilantro, as desired.