1. Stir together first 2 ingredients. Place a large cast-iron skillet over medium-high heat until hot. Add half of corn mixture; cook, stirring constantly, 4 minutes or until vegetables begin to char. Transfer mixture to a 3-qt. saucepan. Add remaining corn mixture to skillet; cook, stirring constantly, 4 minutes or until vegetables begin to char. Stir in cumin and coriander; cook, stirring constantly, 2 to 3 minutes or until fragrant.
2. Add 2 cups broth to corn mixture in saucepan, and process with a handheld blender 1 to 2 minutes or until slightly smooth. Add remaining corn mixture and 2 cups broth to saucepan; bring to a light boil over medium heat. Reduce heat to medium-low, and simmer, stirring often, 5 minutes. Stir in salt and pepper.
3. Whisk together sour cream and cornmeal in a heatproof bowl. Whisk in 1/2 cup hot soup. Add sour cream mixture to soup. Simmer, stirring occasionally, 5 minutes or until thickened.