Season the beef shanks with the salt and pepper.
Set the instant pot to Saute. Add the oil. Sear the beef shanks in single layer, on both sides (around 3 minutes on each side). Carefully remove them to a plate.
Saute the aromatics (onion, celery, carrot, garlic) for 3 minutes, stir to make sure they don't stick to the bottom of the pot.
Add back the beef shanks. Add the tomato paste, canned diced tomato, thyme, chicken stock, white wine.
Cancel the Saute Function. Close and lock the lid. Set pressure cooker on high for 32 minutes, with natural release.
In the meantime, prepare the Gremolata. Zest the lemon. Chop the parsley. Mince the garlic. Mix everything together and set aside.
When the pressure cooking is done and steam is released, open the lid. Remove the beef shanks and keep warm. Taste the sauce, skim off the fat on the surface, set on Saute to slightly reduce the sauce.
Sprinkle the gremolata on top of the Osso Buco. Serve with risotto, rice or pasta.