To make it, jalapeños and serrano chiles are cooked down with onions in butter until soft. Adding flour creates a roux that will help to keep the sauce thick and emulsified. The base for the sauce is milk, but you could just as easily use an equal amount of chicken stock for a more savory, less creamy dip, or tomato juice for a hyper-tomato-ey one. Next come the tomatoes and cilantro which add a necessary tang and bite, respectively, to cut through the dip's richness—if you're not a "cilantro person," you can leave it out. Add the cheese 1 handful at a time, stirring until it's completely melted. This ensures an even texture, a sauce that doesn't break, and renders the waiting tortilla chip-holders impossibly restless.