Take a few sprigs of fresh parsley, cut off the stems and set aside. Finely mince the leaves.
In a large saucepan, lightly sauté the garlic, parsley stems in olive oil just until the garlic begins to give off its fragrance. Add the mussels, give them a turn, then pour in a good glug of white wine.
Cover and let the mussels steam until they have all opened, shaking the pan from time to time. It should only take a minute or two. (If any fail to open, discard them.)
Remove the mussels to a bowl and reserve their cooking juices in the pan until needed.
In another large saucepan or sauté pan, gently sauté the second garlic clove (and the peperoncino if using) in olive oil. When the garlic begins to give off its aroma, add the tomatoes and let them simmer until they have reduced down to a saucy consistency but not entirely melted, perhaps 5-10 minutes, depending on the type and quality of tomato you're using.
Pour the reserved juices from the steamed mussels into the sauce, making sure that you avoid adding any slit that may have settled at the bottom of the pan. (If there's a lot, you can pour the juices through a colander lined with cheesecloth, just to be on the safe side.) Stir and let the sauce once again reduce down a bit. Add a pinch of the minced parsley, then taste and adjust for seasoning. (Since the mussel juices are quite briny, it may not need much or perhaps any seasoning.)
Add the mussels into the pan and toss them with the sauce. Simmer just for another minute or two to rehead the mussels, tossing so they are well covered with the sauce.
Serve right away, sprinkled with minced parsley, with a good crusty bread on the side