Mexican Corn-and-Poblano Soup

INGREDIENTS

• 3 scallions
• 1 large white onion, quartered lengthwise, root end kept intact
• 2 large fresh poblano chiles (8 ounces)
• 1 1/2 pounds frozen corn kernels, thawed and drained (about 3 1/3 cups), divided
• 1 tablespoon coarse salt
• 4 cups plus 3 tablespoons water, divided
• 2 tablespoons fine cornmeal or masa harina
• Queso fresco cheese, crumbled, for sprinkling