In a large pot or Dutch oven over medium heat, cook the ground beef with the diced onion until the beef is browned and the onion is soft, about 6–8 minutes. Break the meat into small pieces as it cooks.
Add the garlic and diced poblano (or jalapeño) and cook, stirring, for 2 minutes until fragrant.
Stir in the fire-roasted tomatoes, Southwest seasoning, green chiles, broth, corn, and beans. Bring to a gentle boil over medium-high heat. Reduce the heat to medium-low, partially cover, and simmer for about 20–25 minutes, stirring occasionally, until the flavors meld.
Taste and adjust salt as needed. Serve hot, topped with sour cream, chopped cilantro, shredded cheese, and avocado if desired.