Set Instant Pot to Sautée mode and allow to preheat.
Meanwhile, rinse leg of lamb and pat very dry with paper towel. Cut into 2" chunks and season all sides with a bit of kosher salt.
Add avocado oil to preheated Instant Pot and brown all sides of the lamb in batches. Transfer browned meat to bowl and set aside.
To the Instant Pot, add onion, celery, and carrot - season with a pinch of salt. Sautée vegetables until softened (approximately 10-12 minutes). Be sure to scrape all the brown bits off the bottom of the pot.
Add garlic, rosemary, oregano and bay leaves and season with 1 tsp of freshly-cracked black pepper. Sautée an additional 60 seconds.
Return browned meat to the Instant Pot and add tomato passata and chicken stock. Give everything a stir and cook on Sautée mode until the sauce comes to a simmer. **Note: this will help reduce the time it takes to seal the Instant Pot and Initiate the timer.
Close the lid, set the pressure release valve to the 'Sealing' position and program the Instant Pot to Manual Mode, High Pressure for 60 minutes. **Note: You can also use the 'Meat/Stew' mode with High Pressure for 60 minutes.
After cooking time has elapse, depressurize manually, and switch back to the Sautée mode to reduce the sauce. As the sauce reduces, use a pair of tongs and a fork to shred the meat in the pot.
Reduce sauce to desired consistency (I like it thick and chunky), taste for seasoning and adjust salt and pepper as required. Note: If you reduce the sauce too much, simply add a little bit of stock to reconstitute.
Serve over Cauliflower Polenta or Zucchini Noodles.