Pat veal shanks completely dry with paper towels. Season generously with salt and pepper, then dredge in flour, shaking off excess. This dry surface is essential for a good sear.

Set Instant Pot to Saute on High. Add 1 tablespoon olive oil. When shimmering, add shanks in a single layer (work in batches if needed). Sear 4 minutes per side until deep golden brown. Transfer to a plate.

Add remaining 1 tablespoon oil to pot. Saute onion with a pinch of salt until softened and lightly golden, about 5 minutes. Add garlic and cook 30 seconds until fragrant. Pour in wine and scrape up all browned bits from the bottom with a wooden spoon. Simmer 2 minutes until slightly reduced.

Stir in broth, tomatoes with juices, thyme, and bay leaf. Nestle shanks into liquid, making sure they are mostly submerged. Lock lid, set valve to Sealing, and cook on High Pressure for 35 minutes. When done, allow 10-minute natural release, then quick release remaining pressure.

Carefully transfer shanks to a plate and tent with foil. Remove and discard bay leaf. Stir rinsed rice and 1.5 cups water into the hot braising liquid. Lock lid, set to Sealing, and cook on High Pressure for 6 minutes. Quick release immediately. Stir in butter and let rest 3 minutes. The rice should be creamy and tender with a slight bite.

While rice cooks, combine parsley, lemon zest, and minced garlic for the gremolata. Spoon rice onto plates, top with shanks, and sprinkle generously with gremolata. Serve with the marrow from the bones if desired.