Preheat the oven to 450 F.
Drain the pork and pat dry with paper towels. Set aside while you prep the herb mixture and oven heats up.
In a small bowl (or using a mortar and pestle), crush together the olive oil, rosemary, sage thyme (and dried herbs, if using), and garlic into a paste (I keep a jar of freshly ground garlic in oil. I always have it on hand for when I need a little bit of ground garlic).
Using a sharp knife, score the fat (but not the meat) making 1/8-inch-deep cuts about 1 inch apart.
Rub the pork loin with salt and pepper and place it fat side up in a foil lined roasting pan or a foil lined rimmed baking sheet.
Add one cup of water to the bottom of the pan. Place the pan on the middle rack in the oven (you may need to remove the top rack of oven like I did to make it fit in).
Roast until some fat has rendered, about 30-45 minutes.
After 30-45 minutes, carefully remove the pan from the oven.
Carefully rub the pork with the herb paste mixture (the pork will be hot, so you could use a spoon to rub it on) and carefully place the sliced onion under the pork.
Completely cover the pork tightly with heavy-duty foil.
Place back in the oven and reduce the heat to 325 F.
Continue to cook for 4-5 hours until the meat is very tender and the skin is crispy, or until the internal temperature as measured with a meat thermometer is 160° F (70°C) – medium; 170°F (75°C) – well done.
It takes about 22 minutes per pound for medium done. For pork more well-done, cook it for 27-30 minutes per pound. Mine was ready in about 4.5 hours (I had an 8 lb roast).
Transfer the pork to a large bowl or serving plate. Let rest for about 30 minutes.
When cool enough to handle, use two forks or your fingers to shred the meat into pieces. The hard outer part part can be lifted right off and shopped into large pieces (crispy skin!). The rest of the meat should be tender enough to pull apart with forks. Shred, adjust seasoning if necessary, and serve!
Or transfer to a cutting board and slice into large chunks (which is what I did). Remove any large pieces of fat or bones.
Make a sandwich: Use the shredded pork to make sandwiches (that's what we did for Christmas). Butter some brioche buns in a pan with melted butter. Toast both sides of the bun until they're golden. Serve with the shredded pork and barbecue sauce. Add in some coleslaw. It is messy and delicious!