Unless you methodically worked your way across the menu how would you know what other treasures it held? Friend of the Food Chain Rob Lopata applied Standard Ordering Procedure to suss one out. On a recent visit he noticed the majority of customers were huddled over steaming bowls of beef soup. Caldo de res doesn't get much mention in the broader literature either (in English anyway). Diana Kennedy doesn't bring it up in any of her books, and neither does Bayless. There are plenty of digital recipes but little information on its provenance in the universe of regional Mexican cooking. Maybe that's because it's so elementary—stock, beef, and an assortment of vegetables. What culture (excepting Hindus and Chinese Buddhists) doesn't have a beef soup in its history? What else are you going to do with the tough, bony cuts of beef that won't grill well?
At Tio Luis, they're using chamorro, or beef shank, which produces a murky broth, thin but not without body, and plenty beefy. They must be cooking it long, low, and slow, because you can pull apart the substantial chunks of meat with little effort, the better to pile on the accompanying corn tortillas and then douse with salsa. But what's more apparent is that the fat hunks of vegetables—chayote, carrots, cabbage, green beans, potatoes, and corn—are cooked separately to just the point where they soften but still maintain structural integrity, before they're added to the soup. The broth is minimally seasoned, plenty salty, but absent chiles or anything else to distract from its clean, beefy flavor. If you're feeling spunky, dump in the chopped cilantro and onion that comes on the side.
Tio Luis Tacos, 3856 S. Archer, 773-468-2267