Heat the oil in a pot over medium heat. Add the onion and bell pepper and cook for 3 to 4 minutes. Add the garlic, chili powder, cumin, oregano, and chili flakes and cook 1 to 2 minutes more. Add the remaining ingredients, except the green onions, and bring to a gentle simmer. Simmer 10 to 15 minutes, stirring occasionally, until yams are just tender. Adjust the seasoning and sprinkle servings of the stew with chopped green onions.