The mussels can be prepared through Step 3 and refrigerated for up to 4 hours.
Buy mussels with unbroken, tightly closed shells. To prepare, rinse them under cold running water and rub them with a stiff brush if they seem gritty. Just before cooking, remove the fibrous threads (known as the beard, or byssus) that protrude from the shells, grasping them with a kitchen towel and pulling firmly. Tap all open mussels, and if they do not close, discard them. After cooking, all the shells should open; discard any mussels that remain closed.
If you cannot fit all the mussels in your pot at one time, steam them in batches.
All that's needed to complement the strong flavors of this pesto is a palate-cleansing, lean dry white for contrast. Stick with an Italian Pinot Grigio.