Mussels with Fennel and Lovage

Name: Mussels with Fennel and Lovage
Ingredients: 4 pounds mussels, scrubbed, debearded; 1 fennel bulb, trimmed, cut into 1 cubes or thinly sliced; 1 cup verjus (or 1/4 cup apple cider vinegar and 3/4 cup white grape juice); 1/3 cup dry white wine; 1/4 cup (1/2 stick) unsalted butter, cut into 1/2 pieces; 1 cup lovage leaves or celery leaves, divided; Kosher salt and freshly ground black pepper; Ingredient info: Verjus is available at specialty foods stores, chefshop.com, and terrasonoma.com.