The Best Part of This Sonoran Cheese Soup Is the Treasure at the Bottom of Each Bowl

Caldo de queso is an undersung regional Sonoran dish that’s deeply satisfying yet surprisingly light. The cheese soup features chunks of potato simmered in a broth with tomatoes, roasted green chiles, and evaporated milk until soft, with fresh Mexican cheese such as queso Quesadilla added to each bowl. The gooey, melty globs of the cheese at the bottom of each serving bowl are perhaps the soup's greatest treasure. The cheese mostly keeps its shape but softens and begins to break apart for a satisfying chew that’s characteristic of the dish. 

Caldo de queso is popular throughout the area straddling the northern border of Mexico and the southern border of Arizona. But what was once a highly localized recipe can now be enjoyed by the rest of the world, thanks to wider spread access to ingredients like Mexican oregano, varieties of cheeses, and green chiles. Here are a few tips for making it at home, no matter where you live.

Seven Tips for Cheesy, Bright Tasting Caldo de Queso

1. Don’t sauté the aromatics. I know, a lot of great soup and sauce recipes start with sautéing aromatics like onion and garlic to intensify their flavor, but this isn’t the case with caldo de queso. Here, adding the raw onions, tomatoes, and garlic and canned green chiles to the liquid to simmer adds a bright, acidic flavor to the soup, which helps the soup taste remarkably light, considering it’s also filled with hearty potatoes, cheese, and evaporated milk.
2. Stick with readily available canned chiles. There are thousands of varieties of fresh green chiles, but if you live outside of the Southwest, they likely aren’t available year round. So for this recipe, look for packaged, diced, roasted hot or mild Hatch, New Mexico, or Anaheim chiles that are often simply labeled “green chile peppers” or “green chili.” They’re available at most grocery stores, so they are convenient to use year-round. (Here's one example from Kroger.) 
3. Skip fresh dairy and use evaporated milk. Evaporated milk has been used for making caldo de queso since the late 1800s when new canning processes and railroad access meant that shelf-stable milk could be found in the Sonoran Desert—no refrigeration needed. The dairy adds creaminess and subtle sweetness to the soup. Do not to confuse evaporated milk with condensed milk, which is very sweet and used in desserts. 
4. Pick the right potato. I prefer to use Yukon Gold potatoes here. They have a creamy texture, medium starch content, and thinner skins that don’t need to be peeled. Leaving the skins on also helps the potatoes retain their shape while cooking.
5. Simmer the potatoes until tender, then add the dairy. The potatoes not only add substance to the soup, the starch from the potatoes ensures the soup won’t split or separate when the dairy is added. You see, while the fresh tomatoes and green chiles add bright, tart flavor to liven the soup up, they also lower the pH of the soup. The slightly acidic broth can cause a chemical reaction when the dairy is added, causing the proteins in the evaporated milk to separate. To avoid any risk of this happening, the potatoes are simmered until tender in the broth, so they release their starches into the soup. When the evaporated milk and starch from the potatoes simmer together, the starch molecules from the potatoes swell up and adhere to the milk proteins in the evaporated milk to retain a smooth and emulsified broth base.
6. Save the cheese for serving. Once the soup is prepared, the shredded cheese is added to the bottom of individual serving bowls to keep it from fully melting or separating. When the hot soup is ladled over the soup, the heat gives the cheese the signature pleasantly gummy, chewable texture that's essential for a proper caldo de queso. Using a Mexican cheese variety such as queso Quesadilla, Oaxaca, or even a packaged Mexican cheese blend that melts easily is key.
7. Finish it with a slightly spicy garnish. I recommend finishing the soup with crushed dried chiltepin chiles. It’s how it’s served in Arizona, and the gentle heat of the dried chiles complements the subtle flavors of the soup.