Grate cheeses and place in a large bowl, sprinkle with cornstarch, and toss to coat. Transfer cheese mixture to a large saucepan and add milk. Set over low heat and cook, stirring occasionally, for about 10 - 15 minutes or until mixture is smooth and melted.
Chop onion and stir into a bowl with the 4 ounce can of chiles (including juices) until well combined.
Best if served directly from a slow cooker or saucepan kept on low.
