Set the pork butt out on the counter an hour before you plan to cook, if possible. This will help the crust sear well (but don’t stress if you’re pressed for time, there is enough fat on there to get caramelization either way).
Trim the pork butt of any excess fat you wish to remove. Generously salt and pepper the pork on all sides. I use about 1 tablespoon of salt and 1 teaspoon of pepper.
Heat a large, heavy-bottomed saute pan over medium-high heat and add the oil.
Using tongs to rotate the meat, sear the pork on all sides for 5-7 minutes until a deep brown crust forms. Remove from the heat.
To a large slow cooker, add the stock and worcestershire and whisk together.
Add the pork in and spoon the onions and garlic over and around the pork.
Cook on low for about 8 hours, or until the pork shreds easily with a fork. It should be fall-apart tender when you lift it up.
While the pork is cooking, make the parsley-vinegar sauce by combining all of the ingredients in a small bowl. Stir to combine and season with salt and pepper to taste. Set aside, covered, until ready to serve.
When the pork is cooked, transfer to a wooden cutting board. Shred it up with two forks, removing any extra fat you want to get rid of and discarding that. Spoon the slow cooked onions and garlic over the top of the pork evenly.
Plate the hunks of pork on serving dishes, spoon the parsley-vinegar sauce over top, and serve. I like to serve this with a simple roasted green vegetable or salad and roasted potato wedges.