I recently had these mussels which were OUT OF THIS WORLD, and the waitress shared some of the ingredients with me - I will be experimenting to get it right. She said the secret ingredient is Anise Seed - gives it a bit of sweetness. Also, unsalted butter, sea salt, fresh basil, white wine, garlic, lemon (I assume the zest), mussel broth, of course the tomatoes and red onions you mentioned. If you come up with the right combo, I would love to hear from you. I will share same after I try to make.