In a small fry pan over medium heat, toast the fennel seeds and red pepper flakes, stirring frequently, until fragrant, about 1 minute. Transfer to a small bowl and add the garlic, espelette, rosemary, lemon zest, allspice and sugar. Set aside.

Place the pork belly, skin side down, on a cutting board. Using a sharp knife, score the belly flesh (pork loin) in a checkerboard pattern about 1/3 inch deep so the roast will cook evenly. Flip the belly so it is skin side up. Using a paring knife, poke dozens of holes, each 1/8 inch deep, through the skin all over the belly. Don't be gentle, keep poking! Using the jagged edge of a meat mallet, pound the skin all over for 3 minutes to tenderize it, which will help make the skin crispy when roasted.

Generously season the skin side of the pork belly with salt and rub with half the spice mixture. Flip the pork belly so it is skin side down, rub the remaining spice mixture onto the loin side and season generously with salt. Roll the pork belly with the skin side out and secure with kitchen twine around the loin; tie crosswise with kitchen twine at 1/2-inch intervals. Trim the twine. Transfer the roast to a wire rack set on a rimmed baking sheet.

Refrigerate roast, uncovered, for 1 to 2 days to allow skin to air-dry; pat occasionally with paper towels. Let porchetta sit at room temperature for 2 hours before roasting. 

Preheat the oven to 500°F. 

Season porchetta with salt. Roast on wire 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 a thermometer inserted into center of meat registers 145°F, 1 1/2 to 2 hours more. If skin is not yet deep brown and crispy, increase heat to 500°F and roast for 10 minutes more. Let rest for 30 minutes before serving. Using a serrated knife, remove kitchen twine and slice porchetta into 1/2-inch rounds.

NOTE: The French red pepper spice, espelette or Piment d'Espelette, adds a slightly smoky flavor, mild heat, and robust peppery taste unmatched by other spices. Espelette can be found in most specialty foods grocery stores. Otherwise, smoked paprika may be substituted.