Season the beef short ribs with salt and black pepper. In a large skillet, heat the olive oil over medium-high heat. Sear the short ribs on all sides until browned, about 3-4 minutes per side. Remove from skillet and set aside.
In the same skillet, add the chopped onion and garlic. Sauté for about 2-3 minutes until the onion is translucent.
Pour in the beef broth, red wine, Worcestershire sauce, and tomato paste. Stir to combine, scraping up any browned bits from the bottom of the skillet.
In a slow cooker, place the seared short ribs, sautéed onion and garlic mixture, thyme, rosemary, bay leaf, carrots, and celery. Pour the liquid mixture over the top.
Cover and cook on low for 8 hours or on high for 4 hours, until the meat is tender and falls off the bone.
Remove the bay leaf before serving. Serve the short ribs with the vegetables and sauce spooned over the top.