Set the Instant Pot to sauté and add oil to the insert.
When hot, add lamb shoulder and brown on both sides, then remove, place on a plate and set aside.
Turn off the Instant Pot, add stock and scrape the bottom of the insert to deglaze.
Then mix in the anchovies and garlic purée.
Put the lamb back in the Instant Pot insert, sprinkle the oregano and salt on top and add in the rosemary sprig.
Place lid on the Instant Pot, close and turn the valve to the sealing (not venting) position.
Set the Instant Pot to manual/pressure cook, high pressure for 90 mins.
When cook time is done, allow for at Natural Pressure Release (NPR) of 15 mins and then perform a quick pressure release (QPR) to remove remaining steam.
Slice, garnish and serve.