Preheat the oven to 350 degrees F. Heat the oil in a Dutch oven or large ovenproof pot set over medium-high. Brown the lamb on all sides. Add the onion, carrot, bell pepper and garlic and cook 2 to 3 minutes more. Stir in the flour, tomato paste, paprika, cumin, fennel seeds and chili flakes and cook 1 minute more. While stirring, slowly mix in the beef stock. Mix in the orange juice and bring to a simmer. Cover and cook in the oven 45 minutes. Mix in the potatoes and cover and cook in the oven 45 minutes more, or until the lamb and potatoes are tender. Mix in the mint, season with salt and pepper, and serve.