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1
Heat the oil in a nonreactive 2-quart saucepan over medium heat.
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2
Add the onion and cook until translucent, about 10 minutes.
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3
Add the garlic and cook until lightly browned, 5 more minutes.
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4
Add the chile flakes and tomatoes, turn down the heat to low, and simmer for 30 minutes.
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5
Add the salt and pepper.
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6
Remove from the heat, puree in a food processor, and, if you want a smoother sauce, run the puree through the small-holed plate of a food mill.
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7
Add the lemon zest, return the puree to the saucepan, and set aside.
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8
Combine them with the wine, garlic, thyme, bay leaf, and peppercorns in a large nonreactive pot with a tightfitting lid.
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9
Bring to a boil over medium heat, cover, and cook until all the mussels have opened, about 5 minutes.
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10
Remove from the heat and discard any mussels that have not opened.
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11
When they are cool enough to handle, separate the mussels from their shells, discarding half of each shell.
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12
Put the mussels in the tomato sauce, add the chives, and reheat gently, just until hot.
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13
To serve, line a platter with the rock salt and arrange the empty shells on top.
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14
Spoon a mussel and an ample amount of sauce into each shell.
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15
Serve immediately, garnished with the lemon half.
