How to Make It
Set Instant Pot to “sauté” with timer set to 25 minutes, and let heat 5 minutes. Sprinkle shanks with salt and pepper. Add oil to Instant Pot, and add 2 shanks. Cook, undisturbed, until deep brown, about 5 minutes. Flip, and cook 2 minutes. Transfer to a plate. Repeat process with remaining 2 shanks.
Add butter to pot. Add onion, celery, and 2 teaspoons of the garlic to melted butter; cook, stirring constantly, until softened, 3 minutes. Add vinegar and water; stir, scraping brown bits from bottom of the pot.
Stir in tomatoes, broth, rosemary, and thyme to combine. Place shanks in sauce; secure lid. Turn steam valve to “sealing” position, and press cancel. Cook on high pressure 40 minutes. Carefully release steam.
While veal cooks, combine parsley, zest, and remaining 1 teaspoon garlic in a small bowl. Skim off and discard as much fat from osso buco as possible. Transfer shanks to plate; cover to keep warm.
Set Instant Pot to “sauté”, and bring liquid to a simmer. Cook until reduced and slightly thickened, about 10 minutes. Serve shanks over puréed cauliflower with simmered sauce; top with gremolata.