Pat short ribs dry and season with 1/2 teaspoon each salt and pepper. Set Instant Pot to Sauté. Add oil and, in batches, brown ribs, adding more oil if needed; transfer to bowl.Add carrots, celery, onion, and garlic to pot and cook, stirring often, until just tender, 6 to 8 minutes. Push vegetables to edges of pot, add tomato paste to center, and cook, without stirring, until browned, 1 to 2 minutes.Add wine and simmer until reduced to 1/2 cup, about 15 minutes. Stir in crushed tomatoes, Worcestershire sauce, rosemary, thyme, and then broth. Press Cancel. Lock lid and cook on high pressure 35 minutes. Use natural release method for 10 minutes, then release any remaining pressure.Transfer short ribs (and any loose bones) to bowl; discard rosemary and thyme. Skim off and discard any fat from liquid. Using immersion blender (or standard blender), puree sauce until smooth. Shred meat into large chunks, discarding bones, and stir back into sauce.Prepare polenta: Bring milk, butter and 1 tsp salt to a boil. Whisk in polenta. Cook, stirring constantly, until mixture is thick and creamy and comes away from side of pan, 3 to 4 minutes. Stir in cheese and, if too thick, 3 to 4 tablespoons water. Serve short ribs and sauce over polenta. Sprinkle with gremolata if desired.