For Pho Bo, first bring leg slices to a simmer in a pot, covered with cold water. Boil for 5 minutes, drain and rinse thoroughly with cold water.
Cut the brown onions in half. Line a pan with aluminum foil, place the onion halves cut side down in the pan. Cook over high heat until well browned.
Slice ginger, unpeeled.
Place star anise, cinnamon, cloves and peppercorns in a tea strainer or small piece of muslin cloth.
Place the leg slices in the pot again and cover with 3 liters of water. Add the onions, ginger and spices (in a strainer or sachet) and bring everything to a boil. Cook for 2 1/2 hours over medium heat, occasionally skimming off any foam that forms with a skimmer. If too much liquid boils away, add water (up to 1 quart).
While soup is cooking, soak rice noodles in cold water for 30 minutes. Drain rice noodles. Then boil in plenty of salted water for 1 minute, again drain and rinse. Mix with the sesame oil.
Cut the beef fillet into slices as thin as possible. Peel the red onion and cut into very fine slices. Wash the herbs, shake dry and pluck off the leaves. Rinse the mung bean sprouts in a sieve and drain. Wash the chili peppers and cut into rings. Cut the lime into eighths. Chill all prepared ingredients.
After 2 1/2 hours of cooking, remove the leg slices from the broth. D