Pat the beef cubes completely dry and toss them with the salt and pepper.
Heat the oil in a heavy pot over medium-high. Brown the beef in two or three batches, leaving space between the cubes, about 4 minutes a side. Set each batch aside.
Lower the heat to medium. Cook the onion and bell pepper in the fat left in the pot, 6 minutes, scraping up the browned bits from the bottom.
Add the garlic, cumin and oregano and cook 30 seconds, then sprinkle in the flour and stir constantly for 2 minutes, until it smells toasted and coats the vegetables.
Add the tomatoes and cook 3 minutes, then pour in the broth gradually while stirring, so the flour disperses without lumps.
Return the beef and any juices. Bring to a bare simmer, where only a few bubbles break the surface, then cover partly.
Cook 1.5 hours at that bare simmer, stirring occasionally, until the beef is nearly tender.
Add the potato and cook 25 to 30 minutes more, uncovered, until the potato is soft and the gravy coats a spoon. Taste for salt and serve with warm tortillas.