In a bowl, toss the shredded cheeses with cornstarch until well coated.
In a saucepan over medium heat, warm the milk until hot but not boiling.
Gradually add the cheese mixture, stirring constantly until smooth and melted.
Stir in salt, chili powder, garlic powder, onion powder, and cumin.
Fold in the diced tomatoes with green chilies and jalapeños if using.
Serve warm with tortilla chips, pretzels, or veggies.