Step 1

Season short ribs with salt and pepper. Heat oil in a large heavy pot over medium-high heat. Working in batches, cook short ribs until browned on all sides, 8–10 minutes per batch; transfer to a plate. Pour off fat from pot.

Step 2

Add pancetta to same pot and cook, stirring occasionally, until fat is rendered and pancetta begins to brown, 5–7 minutes. Add onion, thyme, and carrots. Cook, stirring occasionally, until onion is softened, about 5 minutes. Add wine and tomatoes and bring to a boil. Return short ribs to pot, pressing down to submerge. Cover pot, reduce heat, and simmer until short ribs are very tender and meat is almost falling off the bone, 3 1/2–4 hours.

Step 3

Transfer short ribs and carrots to a large baking dish or platter and cover with foil to keep warm; discard thyme and any bones in pot. Strain cooking liquid through a fine-mesh sieve into a medium bowl; return to pot. Bring to a boil, reduce heat to medium, and simmer until thickened and reduced by half, 15–20 minutes; season with salt and pepper. Return short ribs to pot and turn to coat.

Step 4

DO AHEAD: Short ribs can be cooked 2 days ahead. Cover and chill.

Step 5

Combine parsley, chives, garlic, and orange zest in a medium bowl; season with salt and pepper.

Step 6

Place short ribs, carrots, and sauce on a large serving platter; serve with gremolata.