Preheat your Instant Pot using the SAUTE function (normal) on the display panel. In a bowl combine the sugar, onion powder, sea salt, ground mustard, black pepper, paprika, garlic, and cayenne. Rub the seasoning mixture into the pork butt, coating it all over. When the display reads HOT add the oil and brown the pork butt on all sides.
Remove the pork butt and deglaze the pan with the chicken stock. Use a spatula to loose any stuck-on bits. Layer the onions on the bottom of the pot and place the pork on top. Cover with Worcestershire sauce and barbecue sauce.
Place the lid on the pot and make sure the pressure valve is in the sealing position.
Using the display panel press the CANCEL button. Select the MANUAL/PRESSURE COOK button and use the +/- buttons until the display says 90 minutes.
When the pot beeps to let you know it's finished, let the pot naturally release the pressure for 20 minutes before switching the pressure valve to the venting position. Once the steam has released and the pressure lock has disengaged, open the lid and remove the pork. Place it in a bowl and shred it with forks removing any overly fatty pieces. Serve with extra barbecue sauce.