Trim the lamb shanks of their excess fat.
Press the sauté button on the Instant Pot. When the pot is hot, add 2 tbsp olive oil and the lamb shanks. Sear them for about 7-10 minutes on all sides, then transfer to a plate and set aside. I prefer to sear 2 lamb shanks at a time, so they properly fit in your pot.
When the meat is seared and set aside, add 1 chopped onion to the same pot. Cook for about 5-7 minutes or until the onion is softened and caramelized.
Then add 2 chopped carrots, 2 chopped celery ribs, 3 garlic cloves and cook for another 2-3 minutes.
Stir in 2 tbsp tomato paste and cook for 2 minutes.
Add 2 cups red wine and let it simmer for about 10 minutes or until the wine reduces in half and evaporates the alcohol.
Then add 1 cup beef broth, 1 tsp dried thyme, 1 tsp dried oregano, 1 tsp salt, 2 bay leaves. Stir it in.
Add pre-seared lamb shanks. Try to tuck them in so they are almost covered with the liquid.
Close the lid and set the pressure cooker to manual high pressure for 50 minutes. When done, let the natural pressure release for 10 minutes, then open the lid.
Remove the meat, set it on a plate, cover with foil, and set aside.
Press the sauté button again and let the sauce simmer.
Mix 1 tbsp cornstarch with ¼ cup water and gradually add it to the sauce until the desired consistency.
Let the sauce simmer for a few minutes until thickens.