In a skillet over medium heat, toast the fennel seed and black pepper until fragrant, 1–2 minutes. Transfer to a spice grinder and pulse until coarsely chopped. Mix spice grind with 1 tbsp kosher salt, red pepper flakes, and sage.
Remove 1/4 of the pork belly skin. With the pork belly meat side up and using a sharp knife, score the meat every inch, making a criss-cross pattern. Flip the pork belly over and score the skin horizontally.
With the meat flesh side up, spread the garlic over the meat and into the criss-cross crevices. Repeat with the spice blend.
Starting with the pork belly skin side down on the the removed skin side, tightly roll. Secure with 7 pieces of butcher twine, equally spaced. Place the rolled pork belly on a sheet pan with a wire rack and refrigerate, uncovered, overnight.
Prepare the grill for indirect rotisserie cooking over medium heat (350º-450ºF).
Remove the pork from the refrigerator. Brush skin with olive oil and season with remaining salt.
Mount pork belly on the rotisserie spit and grill over indirect medium heat, 350º–450º F, until the internal temperature reaches 190º F with an instant-read thermometer, about 4– 4 1/2 hours. If the grill temperature drops, add a handful of unlit coals every hour.
Remove from grill and spit. Tent with aluminum foil and allow to rest for 30 minutes. Carve and serve.
