Porchetta

Put belly skin side down; arrange loin in center. Roll belly around loin so the short ends of the belly meet. If any of the belly or loin overhangs, trim meat. Unroll; set loin aside. Toast fennel seeds and red pepper flakes in a small skillet over medium heat until fragrant, about 1 minute. Tip spices into a bowl; let cool. Finely grind spices in a spice mill and transfer to a small bowl, along with the sage, rosemary, and garlic; set fennel mixture aside. Assemble porchetta according to steps 1-5. Refrigerate roast, uncovered, for 1-2 days to allow skin to air-dry; pat occasionally with paper towels. Let porchetta sit at room temperature for 2 hours. Preheat oven to 500 °F. Season porchetta with salt. Roast on rack in baking sheet, turning once, for 40 minutes. Reduce heat to 300 °F and continue roasting, rotating the pan and turning porchetta occasionally, until an instant-read thermometer inserted into center of meat registers 145 °F, 1 1/2-2 hours more. If skin is not yet deep brown and crisp, increase heat to 500 °F and roast for 10 minutes more. Let rest for 30 minutes. Using a serrated knife, slice into 1/2" rounds.