In a large saucepan, stir together oil, onion, garlic over medium heat until onion is softened, about 5 minutes. Add 1 can of beans and mash into onion mixture with a potato masher or a slotted spoon (mixture will be chunky).
Stir in second can of beans, green chilis, corn, carrot, celery, chicken broth, cumin, chili powder and salt. Bring mixture to a simmer. Blend cornstarch into milk and stir into chowder. Bring slowly to a boil, stirring frequently. Reduce heat and simmer 5 minutes, or until corn and celery are tender and mixture thickens. Stir in 1 cup of cheese until melted.
Serve in individual bowls and top with fresh cilantro, if desired.
