Saute garlic and ginger in half of sesame oil for about 5 minutes.
Add beef broth, soy sauce, mushrooms, red pepper and bok choy stalks to broth and simmer for 5-10 more minutes. Add the bok choy greens in the last minute as they’ll cook much quicker than the stalks.
While all this is simmering, prepare noodles according to package directions, drain in cold water.
Salt and pepper both sides of the flank steak. Lightly sear the steak in the rest of the sesame oil, 2-3 minutes per side so that the outside is browned but the inside is still pink. It’s best to undercook it because it will cook a bit more when you add the hot broth. Remove from heat and slice the steak, across the grain, into strips of any size you prefer.
Put several beef strips and a handful of noodles in serving bowls and ladle broth with vegetables over.
Add more soy sauce if needed, to taste.