To prepare the spice mix, put the brown sugar, chipotle powder (if using), smoked paprika, cumin, oregano, chili powder, and salt and pepper to taste in a bowl. To fully blend the spices, give them a good stir.
To season the pork shoulder, transfer the pieces of pork shoulder into a big bowl and cover the pork with the spice mixture. If you'd like, put on some disposable gloves and use your hands to rub the spices into the pork. Ensure that the spice mixture has thoroughly coated all of the pork pieces. Then save the seasoned pork.
To brown the pork, use the high sauté setting on an Instant Pot that is 6-quarts or greater. When the saucepan is heated, add the oil. When the oil begins to shimmer, add the seasoned pork to the saucepan in even layers while working in stages so as not to pack it too full. Sear the pork for approximately one minute on each side, or until a deep brown crust forms. After that, move the browned pork to a spotless dish or platter and carry out the same procedure with the remaining meat.
Sauté: Add the onion and garlic to the saucepan after setting aside all of the seared pork. Only add a little additional oil to the pot if it appears dry. For approximately two to three minutes, or until they are soft and aromatic, sauté the onion and garlic, turning occasionally.
Put everything together: Toss in the mustard, vinegar, worcestershire, stock, and scrape out any pieces from the bottom of the saucepan with a wooden tool. Stir everything up thoroughly. After that, gently return the browned pork pieces and any leftover liquids to the saucepan.
To cook under pressure, cover the Instant Pot and program it to operate at high pressure for 45 minutes. Once the cooking cycle is over, release the pressure naturally in the "vent" position or as directed by the manufacturer, continuing until the steam stops completely.
Shred the pork by taking the pieces out of the saucepan and putting them on a big baking sheet or dish. Shred the meat with two forks or, if preferred, your hands while wearing disposable gloves. Note: It's strongly advised, but optional, to reserve some of the cooking liquid (au jus) in the saucepan to pour over the pulled pork to keep it moist when storing or not serving it immediately.
Serve: After tasting the pork, taste again and add extra salt or pepper to suit your taste. You can choose to cover the pulled pork with the entire amount of BBQ sauce, or you can save some for when the pulled pork is constructed into sandwiches, sliders, etc. Have fun!