In a small bowl combine, buttermilk, garlic, thyme, parsley, peperoncino, smooth mustard, salt and pepper. Set aside in a cool place.
In another shallow bowl mix together cornmeal and flour. Set aside.
Place closed mussels into a large pot (discard any mussels that are cracked or open before cooking). Cook on medium high for about 5 minutes. Remove mussels from pot as they open, so they don't over cook. Discard any mussels that don't open. Once cool enough to handle remove mussel meat from shells. Discard the shells. Pat dry mussels between a few sheets of paper towel. Discard paper towel.
In a medium sized bowl, combine lemon juice, olive oil, and course mustard. Mix in lettuce and a bit of salt. Set aside.
Heat oil on medium high in a large pan, until it reaches 370F. While pan is heating, line a plate with a few fresh sheets of paper towels.
Remove garlic slices from buttermilk mixture. Dip a few mussel meat into buttermilk mixture and then into cornflour mixture. Fry mussels in small batches for 1-2 minutes, until golden brown. Remove cooked mussels from hot oil with a slotted spoon, placing onto plate lined with paper towels to drain. Season with salt.
Assemble sandwich. (I don't particularly like mayo, but if you do, spread some on the bread). Place mussels on one half of bread. Top with tomatoes, dressed lettuce, and finally other half of sandwich bread. Repeat for second sandwich. Serve and enjoy, while mussels are still hot.