Melt butter over medium heat then add the diced onion, poblano pepper and serrano pepper. Cook the for 2-3 minutes. Add the garlic and chipotle peppers and cook for one more minute.
Whisk the cornstarch and milk in a small bowl and then add to the saucepan. Bring the mixture to a boil and stir frequently for 1-2 minutes until it begins to thicken.