2 tablespoons unsalted butter
1/2 cup chopped onion
2 serrano chiles, stemmed, seeded, and diced
1 jalapeno pepper, stemmed, seeded, and diced
2 cloves garlic, minced
2 tablespoons flour
1 cup milk
1/2 cup chopped cilantro
15 ounces tomatoes, seeded and diced
12 ounces cheddar cheese, grated (3 cups)
12 ounces Monterey Jack cheese, grated (3 cups)
1/2 cup sour cream
In a medium saucepan, melt butter over medium low heat. Once melted, fry onion, serrano chiles, and jalapeno pepper until onion begins to soften, about 5 minutes. Stir in garlic and fry until fragrant, 30 seconds.
Add the flour and fry for about 30 seconds before slowly whisking in the milk. Whisk continuously for 3 minutes. Add cilantro and tomatoes and reduce heat to low. Whisk in the cheese, a handful at a time, until melted. Mix in the sour cream and serve hot with tortilla chips.