Season short ribs with salt and pepper. Sear in a hot skillet 2–3 minutes per side until browned.
Add onion to the skillet and cook 2 minutes to soften. Transfer meat and onion to the crockpot.
Stir wine, beef broth, tomato paste, garlic, carrots, celery, bay leaves, and thyme into the crockpot. Cover.
Cook on low 7–8 hours or high 4–5 hours until meat falls off the bone. Remove bones and shred meat.
For thicker sauce, whisk cornstarch and cold water then stir into hot stew and cook 15–30 minutes more.