Dab meat dry. Peel and halve the onion, clean the garlic and cut in half. Clean lemon grass and chilli. Cut the chillies into fine rings. Peel ginger thinly and cut into thin slices. Boil up about 2 litres of water.
Season with coriander seeds, lemongrass, ginger, salt and pepper. Add meat, garlic and onions. Simmer over medium heat for about 2 1/2 hours. Skim the resulting foam from time to time.
Clean, peel and cut the carrots into fine strips. Peel, wash and grate horseradish very finely. Clean, wash and drain the mushrooms. Wash coriander, shake dry and pluck leaves from the stalks.
Wash the sprouts and let them drip off.
Remove meat from the stock and pour stock through a sieve into a pot, bring to the boil. Cook the noodles in it according to the instructions on the packet. Cut meat into very thin slices. Add carrots, horseradish, mushrooms and meat about 1 minute before the noodles are done.
Arrange in small bowls, garnish with coriander and sprouts. Add wasabi.