When you bring your mussels home from the market, discard any that are chipped, broken or otherwise damaged; discard any mussel that is open.
If you are not using right away, cover with damp paper towels in a container that can be tightly closed and store in the refrigerator.
Just before cooking, wash the mussels in several changes of cold water.
Using a small knife or a dry towel, remove the beards by pulling out towards the hinged side of the mussel.
To remove any remaining sand, soak mussels for about 30 minutes in cold water containing a handful of salt and a handful of flour.
Take each mussel from the bowl and using a firm brush, remove any barnacles before placing into another bowl of clear water.
Melt butter in a medium sized pot; saute onions til clear.
Add garlic& stir for another minute.
Add tomato, oregano, salt& pepper and cook until tomato is soft.
Add wine& bring to a slow boil.
Dry the mussels with a clean towel, place into pot, cover & steam for 4 minutes; remove the lid & stir the mussels.
At this point, most of the mussels should be open; replace the lid and steam for another 2 - 4 minutes.
Discard any that don't open.
Divide into serving bowls& cover with broth.
If you increase the serving size for this recipe, don't add too many mussels to the pot at once or stack them on top of each other as it will make it difficult for the mussels to open due to the weight.