Season the short ribs with the salt and pepper and heat the oil in a large skillet over medium-high heat.
Brown the ribs in batches until a dark crust forms on three sides, 8 to 10 minutes per batch, then place them in a 6-quart slow cooker.
Cook the onion and carrots in the skillet until browned at the edges, 5 minutes, then add the garlic and tomato paste for 1 minute.
Pour in the wine, scrape the skillet clean and boil for 3 minutes before adding the mixture to the cooker.
Add the broth, Worcestershire, thyme and bay leaves, cover and cook on low until the meat nearly slips from the bones, 8 to 8 1/2 hours.
Transfer the ribs to a platter, discard the bay leaves and skim the sauce thoroughly.
Boil the sauce in a saucepan until it coats a spoon, 10 to 15 minutes, then spoon it over the ribs.