Do a test roll of the pork belly and make a mark on the skin with your knife so you can remove the parts of the skin that might be inside the porchetta when it is wrapped up. 

Using that knife mark you just made, slice off the layer of skin that would be inside the rolled-up belly. You just want skin on the very outside of the wrapped porchetta. 

Score the inside of the pork belly all the way across, every two inches at least an inch deep. Turn the pork belly 90 degrees and score it again, making a crosshatch. This is done so that you will have more area for your salt and seasonings to land on the inside of the porchetta.

Flip the pork belly over and make very shallow slices just barely through the skin side. These tiny slices will allow the skin to contract during the cooking process and crisp up. 

Liberally sprinkle kosher salt all over the inside of your pork belly. 

Grind up your whole fennel seed and whole black peppercorns. You can do this with a spice grinder, a blender or even a mortar and pestle. Add ground fennel seed and black pepper corns to a bowl.

To the bowl with the fennel and black pepper, add the red pepper flakes, finely chopped rosemary and sage leaves. Stir to combine. Add pressed garlic and the orange zest. This is the spice blend that will be inside your porchetta.

Rub all the spices into the inside of the pork belly. Try to get the spices into the slices that you cut earlier with your hands.

Roll up pork belly and tie it up with butcher's twine. Roll the porchetta very tightly. 

In a small bowl, mix 2 teaspoons baking powder with 3 teaspoons of salt.

Once the porchetta is wrapped, rub the pork belly on the outside/skin with baking powder/salt mixture. Place wrapped porchetta on a wire rack on a baking sheet. Store in the refrigerator overnight uncovered. This process will help dry out the skin so that it becomes crispier.  

When you are ready to cook, pre-heat the oven to 300 degrees F (148 C).

Roast the porchetta for 3 to 3 and a half hours or until the pork reaches 160 degrees F (71 C).

Remove the porchetta from the oven and let sit aside while you raise the oven temperature to 500 degrees F (260 C).

Sear the porchetta in the very hot oven for 20 to 30 minutes to let the skin brown fully and get crispy.

Remove the porchetta and allow to cool for at least 30 minutes before slicing. If you are making these for sandwiches, the pork will slice a lot better when the pork is fully cooled or refrigerated. 

Slice and serve.