Brown the beef
In a resealable plastic bag, combine the flour, 1 teaspoon salt, and ½ teaspoon pepper. Add the beef and shake to coat evenly; reserve the excess flour mixture. In a large frying pan over medium-high heat, warm 4 tablespoons (2 fl oz/60 ml) of the oil. Add half of the coated beef and cook, turning, until evenly browned, 10–12 minutes. Transfer to paper towels to drain briefly, and then transfer to a Dutch oven or slow cooker. Repeat with the remaining beef, adding extra oil if needed. Sprinkle the browned beef with the oregano.
Finish the chili
Return the frying pan to medium-high heat. Add the garlic and all but about ½ cup (2½ oz/75 g) of the onions and sauté for 1 minute. Sprinkle with the reserved flour mixture and sauté for 1 minute longer. Pour in the broth and chiles. Raise the heat to high, bring to a boil, and stir to scrape up the browned bits on the pan bottom. Pour over the beef. If using a Dutch oven, place over medium-high heat and bring to a simmer. Reduce the heat to medium and cook, stirring frequently, until the beef is tender, 1–1½ hours. (If using a slow cooker, cook on the high-heat setting for 3–4 hours or the low-heat setting for 6–8 hours.) Serve, sprinkled with the remaining ½ cup onion.