Make Ahead Tip: Cover and refrigerate salsa verde for up to 1 day. Or prepare through Step 3, cover and refrigerate the salsa and mussels separately. Finish with Steps 4-6 just before serving.
Equipment: Rock salt, coarse salt or foil
Tips: A microplane grater is a great kitchen gadget that seems to be tailor-made for grating citrus zest. It was originally designed to function as a woodworking tool (called a carpenter's rasp). Its razor-sharp edges shave off the zest effortlessly and make it easier to leave the bitter white pith on the fruit. It's the right tool when you want fluffy, very fine citrus zest. Traditional kitchen graters can be used for zesting citrus, too, but they have a tendency to rip and shred the zest, giving a somewhat more clumpy, wet result.
Discard mussels with broken shells or whose shell remains open after you tap it. Hold mussels under running water and use a stiff brush to remove any barnacles; pull off any black fibrous “beards.” (Some mussels may not have a beard.) Mussels should be “debearded” no more than 30 minutes before cooking.
1 lean meat, 2 1/2 fat