1. Preheat oven to 425 degrees with rack in center. In a food processor, combine garlic, fennel seeds, rosemary, lemon zest, 1 1/2 teaspoons salt, and 3 tablespoons oil; pulse to a paste. Pat pork dry. On a rimmed baking sheet, rub pork all over with garlic mixture and wrap with pancetta, overlapping slightly; secure with twine at 2-inch intervals.

2. Toss potatoes, onion, and lemon slices with remaining 3 tablespoons oil. Season with salt and pepper; scatter around pork. Drizzle with 1/3 cup water; roast, basting with juices, until pancetta is crisp and a thermometer inserted in thickest part of meat registers 138 degrees, 40 minutes. Transfer to a cutting board; tent with foil.

3. Return sheet to oven and continue to roast vegetables, tossing once, 10 minutes more. Remove strings from pork and slice. Serve, alongside vegetables, and drizzled with pan juices.