Step 1

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.

Step 2

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.

Step 3

Assemble porchetta according to steps 1-5 below.

Step 4

Refrigerate roast, uncovered, for 1-2 days to allow skin to air-dry; pat occasionally with paper towels.

Step 5

Let porchetta sit at room temperature for 2 hours. Preheat oven to 500°. Season porchetta with salt. Roast on rack in baking sheet, turning once, for 40 minutes. Reduce heat to 300° and continue roasting, rotating the pan and turning porchetta occasionally, until an instant-read thermometer inserted into center of meat registers 145°, 1 1/2-2 hours more. If skin is not yet deep brown and crisp, increase heat to 500° and roast for 10 minutes more. Let rest for 30 minutes. Using a serrated knife, slice into 1/2" rounds.

Step 6

Set belly skin side down. Using a knife, score the belly flesh in a checkerboard pattern 1/3" deep so roast will cook evenly.

Step 7

Flip belly skin side up. Using a paring knife, poke dozens of 1/8"-deep holes through skin all over belly. Don't be gentle! Keep poking.

Step 8

Using the jagged edge of a meat mallet, pound skin all over for 3 minutes to tenderize, which will help make skin crispy when roasted.

Step 9

Turn belly and generously salt both it and loin; rub both with fennel mixture. Arrange loin down middle of belly. Top with orange slices.

Step 10

Roll belly around loin; tie crosswise with kitchen twine at 1/2" intervals. Trim twine. Transfer roast to a wire rack set in a rimmed baking sheet.