Rinse the mussels and discard all mussels that are open.
Thinly slice the onion and the garlic cloves.
Fry the onion, garlic and rosemary in a little oil in a pot.
Add the mussels and fry for a couple of minutes.
Add the tomatoes and the wine, put on the lid and let it simmer for about 5-7 minutes.
Lift the lid. If the mussels are open they are done.
Serve on a big plate with bread. The mussels that are not opened at this point should not be eaten.