Season the short ribs with salt and pepper.
Heat a skillet over medium-high heat and sear the ribs on all sides until browned.
Transfer the ribs to the slow cooker.
In the same skillet, sauté onions and garlic until translucent.
Add carrots and celery, cooking for another 3 minutes.
Stir in tomato paste, cooking for 1 minute.
Pour in the red wine, scraping any browned bits from the skillet.
Add beef broth and thyme, then bring to a simmer.
Pour the mixture over the short ribs in the slow cooker.
Cover and cook on low for 8 hours, or until the meat is tender.