Step 1

Melt the butter in a saucepan on medium-low heat. Add the onions, serrano chiles, and jalapeño and cook for about 5 minutes or until the onions are translucent. Add the garlic and cook for another 30 seconds. Whisk the flour into the pan and cook for about 30 seconds.

Step 2

Pour the milk into the pan and, while whisking, cook until the sauce is thick, about 3 minutes. Stir in the cilantro and tomatoes.

Step 3

Turn the heat down to low, and ¼ cup at a time, slowly stir in the grated cheese until it’s completely melted. Repeat until all the cheddar and Monterey Jack cheese has been added. Stir in the sour cream and add salt to taste. Serve with tortilla chips.