Heat a 10-inch cast-iron pan over high heat. Add the tomatoes cut-side down, chiles de arbol and serrano chiles. Cook, turning occasionally, until the tomatoes and serranos are charred and dried chiles are fragrant, 8 to 10 minutes. Transfer to a blender. Add the onions, cilantro, salt and 1 cup water. Pulse until slightly chunky. Serve with tortilla chips.
Dried chiles de arbol can be found in the ethnic section at your local grocery store or at specialty Mexican markets.
A viewer or guest of the show, who may not be a professional cook, provided this recipe. It has not been tested for home use.