Preheat the oven to 450°F.
Let me preface this next step by saying that you can have the butcher do the cutting. If you are doing the cutting yourself, first, slice the skin away from the meat and fat in one piece. You will use the skin to wrap up the whole porchetta later. Now unroll and open up the pork belly by cutting into it to create as much surface area as possible, keeping it in one piece. Lay it flat. (You are more or less butterflying it.)
Combine the garlic, rosemary, thyme, fennel, lemon zest, olive oil, salt, and pepper and mix well. Rub down the interior of the porchetta (all of the cut pieces) with the marinade and let sit for an hour.
Roll the porchetta up tightly and wrap it with the skin. Tie up the skin (around the outside) tightly with three pieces of kitchen string to hold the whole thing together. Place the porchetta on a rack fitted into a roasting pan and move it to the center of the oven.
Cook for 45 minutes or until the skin gets nice and crisp. Reduce the heat to 350°F and cook for an additional 2 ½ hours. A thermometer inserted into the center of the pork should read 160°F when done.
Remove the porchetta from the oven and allow it to rest for 20 minutes so that the juices evenly distribute. Slice the porchetta into ½-inch thick rounds. Serve with salsa verde and some good toasted bread to make simple sandwiches.