To prepare the porchetta, place the pork belly, skin side down, and arrange the pork loin inside of the pork belly. Cut and roll the belly around the loin, trimming off any excess that hangs over, to form a neat draping around the loin. Unroll, then assemble as follows: 

Place the pork belly, skin side down, and cut shallow incisions in the meat in a checkerboard fashion to ensure it cooks evenly. Flip the belly over and, using a paring knife, vigorously poke shallow holes in the pork belly skin (this creates airflow and helps to crisp the skin). Then, using a meat mallet, vigorously pound the skin for 2 to 3 minutes (this is optional but helps to produce the crispiest skin).

Generously season the underbelly of the pork belly and the loin with salt and pepper.

In a blender, combine the parsley, sage, walnuts, olive oil, garlic, lemon zest and lemon juice and blend until a smooth, pesto-like consistency forms. Slather the underbelly of the pork belly with the pesto and place the pork loin back inside of it. (Optional chef's step: butterfly the loin and slather it with pesto as well.)

Roll the pork belly around the loin. Securely tie 4 pieces of twine, each 12 inches long, around the belly, spacing the pieces 2 1/2 to 3 inches apart. Trim the twine.

Pat the pork belly skin dry with paper towels. Let the pork stand at room temperature for about 2 hours before cooking.

Preheat the oven to 450°F. Place the pork on a rack in a roasting pan.

Roast the pork for 45 minutes. Reduce the oven temperature to 300°F and continue roasting until the skin is crisp and an instant-read thermometer inserted into the center of the meat registers 130°F, 1 1/2 to 2 hours more. If the skin is still not crispy, turn the oven to broil for a couple of minutes to ensure a brown, crackling crust. 

Transfer the pork to a carving board, cover loosely with aluminum foil and let rest for 30 minutes. This will help the meat stay moist and bring the internal temperature up. 

Cut the pork into rounds and serve with crusty sourdough bread and the fig mostarda (recipe below).

To prepare the fig mostarda, in a sauté pan over medium heat, combine the mustard seeds, coriander and pepper and toast,  stirring frequently to prevent the spices from burning, until fragrant, about 1 minute. 

Add the vinegar and bring to a simmer. Add the figs and cook, stirring occasionally to ensure the mixture doesn't burn, until the mixture reaches a jammy consistency, about 20 minutes. Let cool to room temperature.