Scrub the mussels under cold running water to remove any mud. Using a sharp knife cut away the feathery beards from the shells. Discard any open mussle that does not clamp shut when tapped with a knife. Rinse again in cold water.
Place the lager, onions, garlic, chili, and tomatoes in a heavy-based pan. Bring to a boil. Add the mussels and cook, covered, over a medium heat for about 10 minutes until the shells open. Discard any mussel that has not opened.
Ladle into individual bowls and serve sprinkled with fresh cilantro.