4 pounds Chilean Mussels
1 fennel bulb, trimmed, cut into 1-inch 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-inch pieces
1 cup lovage leaves or celery leaves, divided
Kosher salt and freshly ground black pepper
Heat a large heavy pot over high heat until very hot. Add mussels, fennel, verjus, and wine. Cover and cook, stirring frequently, until mussels are heated through. Using a slotted spoon, transfer mussels and fennel to a serving platter.
Simmer liquid in pot for 1 minute; whisk in butter and 1/2 cup lovage leaves. Season to taste with salt and pepper (and additional vinegar if using grape juice). Spoon over mussels, leaving any sediment behind. Garnish mussels with remaining 1/2 cup lovage leaves and serve immediately.