How to Make It
In a food processor, mince the pine nuts with the garlic and 3/4 teaspoon of salt. Add the 2 cups of mint and the parsley and pulse until the leaves are coarsely chopped, scraping down the side of the bowl once or twice. With the machine on, gradually add the olive oil and process until a coarse paste forms. Season with salt and pepper.
Preheat the oven to 375°. In a large pot, combine the mussels and wine. Cover the pot tightly and cook over high heat; as the mussels open, use a slotted spoon to transfer them to a bowl. Let cool slightly.
Discard 1 shell from each mussel and loosen the meat in the remaining shell. Set the mussels on the half shell on a baking sheet. Lift each mussel and stuff 1/2 teaspoon of pesto under it, then lightly press the mussel back in place.
Bake the mussels for about 10 minutes, or until they're hot to the touch and the pesto begins to bubble. Serve the mussels on plates or on a platter lined with mint sprigs.
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.