Rinse mussels under running water and brush clean. Discard broken mussels and any that are already open. Gently knock any gaping mussels on a wooden board several times and discard those that don't close.
Finely dice the garlic and half the onion.
Combine diced garlic, onion, and 200 ml white wine in a large pot and bring to a boil over high heat.
Add mussels to the pot, cover, and boil for 10 min.
Discard any mussels that did not open.
Season the cooking liquid with salt to taste and serve in bowls with the mussels.