Brown the ground beef in a dry heavy pot over medium-high heat, pressing it flat and leaving it undisturbed for 3 to 4 minutes before breaking it up, so it develops a dark crust rather than steaming grey.
Drain off most of the fat, leaving about a tablespoon in the pot.
Add the diced onion and cook 4 to 5 minutes until soft, scraping up the browned bits from the bottom.
Add the garlic and the taco seasoning and stir for 30 to 45 seconds in the fat, until the spices smell toasted. This is the step that keeps the soup from tasting raw and dusty.
Add the tomatoes with their juice, the beans, the corn, the broth and the salt.
Bring to a simmer and cook uncovered for 15 to 20 minutes, until the broth has body and the flavours have settled. Taste for salt before serving.