Cut the beef sirloin or beef round steak into 1-inch cubes. The key to a flavorful carne guisada is getting a good sear on the beef. Those browned bits at the bottom of the Dutch oven contribute a ton of flavor to the gravy. Pat the meat dry with paper towels to ensure a good sear. Season the beef with kosher salt and freshly ground black pepper.
Heat the oil in a large Dutch oven over medium-high heat. Cook the beef in batches, making sure not to overcrowd the pan. Sear each piece until browned on all sides. The goal is to develop those flavorful browned bits, known as the Maillard reaction, at the bottom of the pan. Transfer the seared meat to a plate and set aside.
In the same Dutch oven, add more oil, if necessary, then sauté the diced onions and green bell pepper. Cook until the vegetables are softened and fragrant, about 8 minutes. Add the minced garlic and cook for another minute.
Add the cumin, chili powder, Mexican oregano, and bay leaf to the Dutch oven. Stir well to combine. Next, add the fresh tomatoes and tomato paste. Stir to combine and cook for 3-4 minutes.
Add the beer to deglaze the pot, making sure to scrape up all the brown bits from the bottom. This step helps create the flavorful gravy that’s characteristic of great carne guisada.
Add a cup of water, then return the browned beef to the pot. Stir well to coat the meat in the sauce. Cover and simmer on medium-low heat for about 3 hours, or until the meat is tender and the gravy has thickened.
If the sauce is too thin, remove ½ cup of gravy and whisk in 1 tablespoon of all-purpose flour. Add it back to the carne guisada and stir to combine. Cook until the sauce thickens, about 10 minutes.