Scrub the mussels under cold running water to remove any mud. Using a sharp knife, cut away the feathery "bears" from the shells. Discard any open mussels that do not shut when tapped sharply with a knife. Rinse again under cold water.

Place the beer, onions, garlic, chili, and tomatoes in a heavy-based pan. Bring to a boil.

Add the mussels and cook, covered, over medium-high heat for about 10 minutes until the shells open. Discard any mussels that do not open.

Ladle into individual bowls and serve sprinkled with fresh cilantro.