- In a saucepan over medium heat, soften the white parts of the onions, the garlic, ginger and red pepper flakes in 2 tbsp (30 ml) of the oil. Add the broth, water, sweet potato, fish sauce and soy sauce. Bring to a boil and simmer for 10 minutes or until the sweet potato is tender. Adjust the seasoning.
- Add the noodles and cook for 3 minutes or until tender. Add broth, if needed.
- Meanwhile, in a skillet over high heat, brown the steak on both sides in the remaining oil until the desired doneness. Season with salt and pepper. Let rest for 5 minutes on a work surface, and then slice.
- Ladle the soup into bowls. Garnish with the meat, bean sprouts, the green parts of the onions and the cilantro. Serve with lime wedges, if desired.