Combine corn starch and milk in a medium saucepan. Bring to a boil, stirring constantly. Continue stirring and boil for 1 minute or until very thick. Remove from heat. Stir in cheeses until melted. Stir in cumin, chili powder and tomatoes. Serve with tortilla chips.
Serving suggestions: Serve Cheese Dip topped with additional shredded cheese and minced fresh cilantro. For a heartier version, add crumbled, cooked hamburger seasoned with taco seasoning.