Season the short ribs with salt and pepper.
Heat olive oil in a skillet over medium-high heat.
Sear the short ribs on all sides until browned, about 3-4 minutes per side.
Transfer the browned short ribs to the slow cooker.
In the same skillet, add chopped onion and garlic, sauté until softened.
Add beef broth, Worcestershire sauce, and tomato paste to the skillet; stir well.
Pour the sauce over the short ribs in the slow cooker.
Add the carrots, celery, and thyme to the slow cooker.
Cover and cook on low for 8 hours or high for 4 hours.
Check for tenderness and adjust seasoning if necessary before serving.