Melt the butter in a saucepan on medium-low heat, and then cook the onions and peppers for about five minutes or until onions are translucent.
Add the garlic and cook for another minute.
Whisk the flour into the butter, vegetable mix and cook for about 30 seconds.
Add the milk to the pot, and cook on medium, whisking constantly until sauce is thick, about five minutes. Stir in the cilantro and tomatoes.
Turn heat down to low, and a 1/4-cup at a time, slowly add the shredded cheese stirring into the white sauce until completely melted. Repeat.
Stir in the sour cream.
Add salt to taste.
Notes: You can vary which chiles you use to control how hot you want the queso to be. Also, if the sauce is too thick, feel free to add a bit more milk a tablespoon at a time. The key, however, to a smooth sauce is adding the cheese very slowly and not adding any more cheese until the previous addition is completely integrated into the sauce.