Sauté the Aromatics: Set your Instant Pot to “Sauté” mode.
Add the lamb tallow and let it melt.
Add the crushed garlic, diced onion, chopped rosemary, and diced celery.
Cook for 5 minutes, stirring occasionally, until softened and fragrant.
Build the Flavor Base: Stir in the salt, pepper, red wine vinegar, and tomato puree.
Continue to cook on “Sauté” for another 5 minutes, allowing the flavors to meld and scraping any browned bits from the bottom of the pot.
Add Lamb and Broth: Pour in the beef broth and stir well to combine.
Add the lamb shoulder chunks, ensuring they are mostly submerged. If needed, add a little water to just cover the meat.
Pressure Cook: Secure and lock the Instant Pot lid, ensuring the valve is set to “Sealing. ” Select the “Meat/Stew” setting and set the timer for 60 minutes.
Natural Release: Once the cooking cycle is complete, allow the pressure to release naturally for at least 15 minutes before performing a quick release for any remaining pressure.
Cook the Mushrooms: Carefully open the lid.
Set the Instant Pot back to “Sauté” mode.
Stir in the sliced mushrooms and simmer for 10-15 minutes, or until they are tender and the stew has thickened slightly.
Serve: Ladle the hot stew into bowls.
Top each serving with a generous dollop of sour cream and a sprinkle of grated Parmesan cheese.
