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.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.