Porchetta With Roasted Potatoes

Preheat oven to 350 °F and position rack in the lowest position. Toast fennel seeds in a small skillet over high heat until fragrant, about 2 minutes. Transfer to cutting board along with garlic, fennel fronds, salt, and pepper and finely chop together. Transfer mixture to a small bowl and stir in oil. Lay pork belly on cutting board, skin side down. Holding a large sharp knife parallel to the cutting board, split the belly in half, being careful not to cut all the way through the opposite end. Open the split belly like a book and spread fennel-garlic mixture all over. Place pork loin at the end of the skinless side of the belly, and tightly roll the belly around the loin, forming a roast. The skin should almost wrap the roast completely. Using butchers' string, tightly tie the roast at 1-inch intervals. Transfer the porchetta to a large roasting pan and roast for 1 1/2 hours. Remove the pan from the oven and add the potatoes and rosemary. Season with salt and toss the potatoes in the pan juices to coat. Roast until an instant-read thermometer inserted into the center of roast registers 165 °F and potatoes are tender, about 2 hours more. Let the porcetta rest for 20 minutes before slicing.