1. Heat the oil in a nonreactive 2-quart saucepan over medium heat. Add the onion and cook until translucent, about 10 minutes. Add the garlic and cook until lightly browned, 5 more minutes. Add the chile flakes and tomatoes, turn down the heat to low, and simmer for 30 minutes. Add the salt and pepper. Remove from the heat, purée in a food processor, and, if you want a smoother sauce, run the purée through the small-holed plate of a food mill. Add the lemon zest, return the purée to the saucepan, and set aside.

1. Combine them with the wine, garlic, thyme, bay leaf, and peppercorns in a large nonreactive pot with a tightfitting lid. Bring to a boil over medium heat, cover, and cook until all the mussels have opened, about 5 minutes. Remove from the heat and discard any mussels that have not opened. When they are cool enough to handle, separate the mussels from their shells, discarding half of each shell. Put the mussels in the tomato sauce, add the chives, and reheat gently, just until hot.
2. To serve, line a platter with the rock salt and arrange the empty shells on top. Spoon a mussel and an ample amount of sauce into each shell. Serve immediately, garnished with the lemon half.

Beverage pairing: Albariño Valmiñor, Spain. With spice, heat, herbs, and sweetness, this dish has a lot going on. The wine should lay in the background, supporting the flavors with good acidity and purity of fruit. Albariño is always a good bet, and this one will taste like a little extra lemon squeezed over the mussels.