First, you must start with a whole pork belly that has the skin still on it, then season the meat side liberally with salt.
Flip it over and rub some baking soda evenly into the skin.
Put the meat in the fridge with the skin side facing up, uncovered to allow the salt to work it’s magic overnight.
Heat the oven to 400°F.
Take the pork belly out from the fridge and lightly pat the skin with a paper towel to dry it.
Make sure to totally wipe off the remaining baking soda residue from the pork's skin.
Using a razor blade, carefully score the skin to expose the fat, without piercing it. Score in diagonal lines 2 inches apart. Then score in the opposite direction, also 2 inches apart, to create an even diamond pattern in the skin.
Flip the belly over so the skin side is down. Evenly scatter the garlic slices over the meat.
Follow with the chopped rosemary, coarsely ground fennel seed, and lemon zest.
Roll up the belly and tie it together at 3-inch intervals using butcher's twine, making sure the twine is snug but not too tight.
If your pork belly doesn't fit in a standard oven, you can cut it in half to make 2 roasts.
Place the pork belly on a pair of wire racks and roast for about 40 minutes.
After that, reduce the oven temperature to 325°F and roast undisturbed for 3 hours.
Check it's tenderness by using a long thin knife or long wooden skewer to probe the meat — it should be very pliant but not falling apart.
Raise the oven temperature to 500°F and roast until the skin is evenly blistered and crispy, about 5-7 minutes (make sure to keep an eye all the time as the skin easily gets burned while blasting)
Remove the porchettas and let them cool for 30 minutes. Slice as thick as you like
If you’re making sandwiches, 1 1/2 inches is about right and serve.