Remove the pork loin from the refrigerator 1 hour before cooking to allow it to get to room temperature. Cut off the rind if attached, but leave the majority of the surface fat on the roast.
Season the pork roast with salt and pepper on all sides.
Heat up vegetable oil in your Instant Pot on SAUTE setting. Add the seasoned pork loin, and sear for a few minutes on all sides until golden brown. Remove the roast from the Instant Pot.
Add diced shallots and garlic to the Instant Pot, and sauté for around a minute until the onion is soft.
Add vegetable stock and Worcestershire sauce to the Instant Pot. Use a spoon or spatula to scrape the bottom of the pot and remove any pieces of pork or onion that might be stuck. This is really important, or the Instant Pot could not be able to get to pressure. Turn off the Instant Pot.
Place a trivet on the bottom of the Instant Pot, and add the pork loin on the trivet. Add thyme sprigs to the pot.
Seal the lid on the pressure cooker, and cook on MANUAL or PRESSURE COOK setting on high pressure for 20 minutes.
Perform a quick release of the pressure following the manufacturers instructions.
Use a meat thermometer to ensure that the internal temperature of the roast is at least 145 F / 63 C. If the temperature is lower, you can cover the lid of the Instant Pot, and leave the roast to steam for a few minutes, or until the temperature has risen sufficiently.
Remove the cooked pork loin from the Instant Pot. Leave it to rest for at least 20 minutes, or up to 1 hour before you slice and serve it.