To a saucepan add the milk over medium heat. Stir in the onion, serrano peppers, poblano pepper, chipotle pepper and garlic. Mix cornstarch with just enough water so it dissolves, stir into the milk and bring to a light bubble, cook, stirring often until thickened. Remove from heat as soon as it is thickened (you will need to keep an eye on it). If it doesn't thicken to your liking, add a little more cornstarch mixed with milk.

Turn down to the heat to very low and whisk in the sour cream. Once the sour cream is incorporated, add a handful of cheese and stir until melted. Continue to add and handfuls of cheese and stir until all the cheese is used, making sure it is completely melted before adding another handful and ensure the queso is smooth. If too hot, the queso may turn gritty and separate. (see tips below to prevent this).

Stir in the tomato. Remove from the heat and allow the queso to cool, it will get even thicker as it cools.

Slow and steady heat - If you heat over high heat, this heats the liquid too quickly. You want to slowly bring up to a simmer over medium heat or medium-low heat is best. And never, ever boil. Always grate the cheese yourself. Pre-grated cheese has anti-clumping added and this will affect the texture of the sauce.

Cornstarch is highly recommended. If you use flour instead of cornstarch, double the amount (cornstarch has twice the thickening power of flour). I also can't guarantee that any raw flour taste will cook out.

Taste for seasoning and adjust to your taste.

To reheat your queso or cheese sauce, add to a pan and heat low and slow, stirring often.