Gather all your ingredients and give yourself plenty of space to work. There's nothing more frustrating than a cramped workspace when you've got a big hunk of pork flopping around and a slippery knife on the edge of the cutting board. I clear out a big work space in the middle of my coffee table or dining room table.
Here's what you're looking for at the end: a crackly, crisp shell with bubbly, crunchy, salty skin around an insanely juicy core that's tender enough to eat with a spoon, dripping with juices, and aromatic with fennel and herbs.
Toasting black peppercorns and fennel seeds whole will help add complexity to their flavor as chemical compounds undergo complex reactions, breaking down and recombining under the heat of a dry skillet. Toast them over moderately high heat stirring and tossing constantly until lightly browned and aromatic.
Traditional porchetta is flavored with rosemary, sage, and other piney herbs. I personally prefer the flavor of thyme, so I use it. Feel free to use whatever herb you'd like. Garlic, on the other hand, is not optional. Use it, and use a ton.
Score the Meat
Pork bellies are thick. Given a couple weeks, salt and flavorings can penetrate deep into the meat (see: bacon and pancetta). We don't have that kind of time on our hands, so to hasten the flavoring process, score the meat deeply with a sharp, sharp knife with the skin side-down.
Score it along both diagonals for optimal flavor-absorption.
Salt it and Rub it
Salt does more than make your pork salty, during its overnight rest, it'll also penetrate into the meat, altering its structure so that it's able to retain moisture more effectively as well as giving it a slightly bouncier, more resilient texture (think sausage or ham, not rubber ball). Salt your meat very generously (a light, even dusting), then add the rest of your aromatics and rub them deep into the grooves in the meat.
Roll up your porchetta lengthwise (with a full pork belly, you should be able to just barely get the ends of the rind to touch. If you have trouble, don't worry. It's OK if they don't quite meet). If you're fancy, you can then tie up your whole porchetta with a single long piece of twine using butchers knots, but for most of us, regular old double granny knots will do. The easiest way is to lay out foot-long lengths of twine at regular one-inch intervals on top of your cutting board, then lay the roast on top. Working from the outside towards the center, tie your roast up as tightly as you can.
When finished, you should have a nice, even log that's probably way too long to fit in your oven in a single piece (unless you were wise and opted for a smaller piece of belly). We'll deal with that in a moment, but for now, notice how conveniently the pieces of string create indentations in the rind. This will be useful for portioning and slicing the roast later on.
Slice Her in Half
If you want to roast your porchetta all in one go on a single sheet tray, you should slice her in half right now into more manageable lengths. Use a very sharp chef's knife or carving knife and try to do it in a single stroke instead of sawing to give the nicest presentation. For the best results, you'll want to then wrap her very tightly in plastic and refrigerate her overnight to give the salt and baking powder some time to work their magic. You could of course, skip that step if you're in a real rush to get pork in your mouth.
Ready to Roast
The next day, preheat the oven to 300°F and remove the porchetta from the fridge. You can cook both of them side by side on a single rimmed baking sheet with a metal rack set on top of it, or carefully wrap one in foil and plastic wrap and freeze it for later use and roast just one half. Porchetta, because of its high fat content, freezes very well. Just be sure that it's wrapped airtight or better yet, sealed in a vacuum-sealed foodsaver-type bag to prevent freezer burn.
Now all it takes is a bit of time. Roast the sucker until its internal temperature has been above 160°F for at least two hours (it may go as high as 180°F). The goal here is to completely break down all of the connective tissue and structural proteins within the belly so that by the time it's done, it should be spoon tender. This takes both time, and temperature. You should plan on about 4 hours total. You'l want to baste it with dripping fat periodically to give it an even color. Either a spoon or a silicone brush will work fine.
Slice and Serve
To carve, use a serrated knife. It helps to use the indentations from the stings as guides to where you should slice for servings. Your skin should make ear-shatteringly loud crackling sounds as you cut, and if at least 50% of your guests don't gasp as if you've just revealed to them the entirety of One Eyed Willie's booty collection, well then, I can only suggest finding better friends or moving to a more appreciative family. Alternatively, hoard the skin for yourself. The meat should be unbelievably juicy and tender with a nice, salty, bacon-y aroma to it, gloriously fatty, and heavily scented with fennel and black pepper. This is fine, fine eating, my friends.