Sear the Lamb Shanks: Set the Instant Pot to Sauté (High) and add the olive oil. Pat the lamb shanks dry with paper towels, then season them with salt and pepper. Sear the shanks on all sides (about 3-4 minutes per side) until browned. Remove and set aside.
Sauté the Aromatics: Add the onion, carrots, and celery to the pot. Sauté for 3-4 minutes until softened. Stir in the garlic and tomato paste, cooking for another minute until fragrant.
Deglaze with Red Wine: Pour in the red wine and scrape up any browned bits from the bottom of the pot. Let it simmer for 2 minutes to slightly reduce.
Add Remaining Ingredients: Return the lamb shanks to the pot. Add the broth, Worcestershire sauce (if using), rosemary, thyme, and bay leaf.
Pressure Cook: Secure the lid and set the Instant Pot to Pressure Cook (High) for 45 minutes. Allow a natural release for 15 minutes, then quick release any remaining pressure.
Reduce the Sauce (Recommended): Remove the lamb shanks and tent them with foil. Set the Instant Pot to Sauté and simmer the sauce for about 5-10 minutes until thickened. You can stir in 1 teaspoon of butter for extra richness.
Serve: Serve the lamb shanks over mashed potatoes, polenta, or buttered noodles, spooning the rich red wine sauce over the top.