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1
Prepare grill.
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2
Preheat a large griddle or 12-inch cast-iron skillet on a rack over very hot coals.
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3
When griddle or skillet is very hot, add mussels and cook, stirring occasionally if using skillet, until mussels are opened, about 3 minutes.
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4
Discard any unopened mussels.
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5
While mussels are cooking, in a small saucepan heat butter and salsa over moderately high heat until mixture sizzles.
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6
Remove pan from heat and stir in lime juice, salt, and epazote.
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7
Drizzle sauce over mussels.
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8
Heat a dry comal or flat iron griddle over moderately low heat until hot and pan-roast garlic and tomato, turning them occasionally to ensure even roasting, until browned and soft throughout, about 25 to 30 minutes.
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9
Discard garlic skins and tomato stem.
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10
In a heavy skillet heat oil over moderately high heat until hot but not smoking and, using tongs, fry chiles, 1 or 2 at a time, turning them, until puffed and just beginning to brown, about 10 seconds.
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11
Transfer chiles as fried, letting excess oil drip into skillet, to paper towels to drain and, wearing rubber gloves, discard stems.
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12
In a blender puree garlic, tomato, chiles, water, and salt until very smooth.
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13
(Salsa may be made 2 days ahead and chilled, covered.)
