Melt a butter in a medium size sauce pan over medium high heat. Once the butter is melted, add in the onion and peppers (poblano and Serrano peppers). Cook for a 2-3 minutes until they are soft. Then add in the garlic, tomato and cumin.
Mix together the evaporated milk and cornstarch in a separate bowl until the cornstarch is mixed into the milk. Then pour this mixture into the sauce pan with the vegetables. Bring the mixture to a simmer over medium high heat. Stir and heat until the mixture is thickened.
Then remove the pan from the heat and stir in the shredded cheese in small batches until the cheese is melted. Mix until the cheese sauce is smooth.
Then stir in the salt. Taste test and add more salt if needed. The cheese sauce will thicken as it sits as well.
Notes
*Refrigerate the leftovers in an air tight container for up to 5 days. When reheating you will need to add more milk to the cheese to thin it out.