Put a skillet that is big enough to handle the mussels on at medium heat. Swirl in enough extra virgin olive oil to cover all of the bottom. Add the chopped onion and stir to get it spread out and stir it a few more times as it cooks. When the onion has become translucent - about 5 minutes - add the chopped garlic and cook for just a minute or so. Don't let the garlic get brown. Then the tomatoes, salt and pepper, red pepper flakes, paprika and red wine (or white wine or chicken broth). Mix it all together, turn the heat up to medium high and bring it up to a simmer,.
When the the liquids have reduced a little put in the mussels. Stir everything together and stir again a few times as the mussels heat up - about 6-8 minutes. If you are using fresh mussels just cook them until they open up and throw away any that don't open.
In Spain it is almost always served with rice and sometimes with pasta. The Italian in me kicked in and I opted for some pasta with mine and some spinach and tomato salad with feta cheese and a little loaf of Poppop's easy homemade bread for dipping and some Spanish Tempranillo red wine.