2 tablespoons freshly minced garlic (about 6 medium cloves)
2 tablespoons finely chopped fresh sage
1 tablespoon finely chopped fresh thyme
1 tablespoon finely chopped fresh rosemary
1 tablespoon red pepper flakes
1 1/2 teaspoons lemon zest
1 8-9lb piece of boneless pork belly, skin on
5 teaspoons kosher salt, plus more for seasoning
3/4 teaspoon baking powder
2-3 fist size chunks of medium smoking wood such as oak or hickory
To make the rub: Place peppercorns and fennel seed in a cast iron skillet over medium-high heat; toast spices until fragrant, about 2 minutes. Transfer to a spice grinder and process until coarsely ground. Transfer spice mixture to small bowl and mix in garlic, sage, thyme, rosemary, red pepper flakes, and lemon zest.
Lay pork belly, skin side down, on a large cutting board. Score flesh on a diagonal with a sharp knife about every inch. Repeat in opposite direction to create a diamond pattern. Season pork belly liberally with salt. Sprinkle rub evenly across pork belly, using hands to pat rub into meat and cut crevices. Roll pork belly into a tight log lengthwise and tie closed with butcher twine about every inch.
Mix 5 teaspoons of salt and 3/4 teaspoon baking powder together in a small bowl. Rub mixture all over exterior of porchetta.
Fire up smoker or grill to 225°F, adding chunks of smoking wood chunks when at temperature. When the wood is ignited and producing smoke, place the pork belly in the smoker or grill and smoke until an instant read thermometer registers 180°F when inserted into center of meat, about 6 to 8 hours. Remove pork belly from smoker and let rest while preparing the grill.
Light one chimney full of charcoal. When all the charcoal is lit and covered with gray ash, pour out and arrange the coals on either side of the charcoal grate and place a foil pan between the two piles of coals. Cover grill and allow to preheat for 5 minutes. Run spit of rotisserie through middle of pork lengthwise and secure ends with rotisserie forks. Place on the rotisserie, cover, and cook at high heat until skin has crisped all over, about 10-15 minutes. Remove pork from grill and spit, cut off butcher twine, slice, and serve immediately.