These shorts ribs are super tender, fall-off-the-bone. Thicken the gravy if you like and serve with mashed potatoes.
Season short ribs with salt and pepper. Heat oil in a large skillet. Place short ribs in skillet and brown on all sides. Remove short ribs to slow cooker.
Add onion, carrots, celery and garlic to skillet. Saute on medium heat until fragrant, about 5-7 minutes. Add tomato paste and cook for a few minutes.
Add red wine, bring to a boil, then reduce heat to medium-low and reduce wine by three quarters.
Pour mixture over short ribs in slow cooker. Add stock, rosemary, thyme, and bay leaf.
Cook on low for 8 hours, until very tender and meat falls off the bone. To remove as much fat as possible, strain sauce, transfer to a container and refrigerate overnight. If you don't have time to refrigerate overnight, slightly cool sauce, then transfer to a Ziploc bag. Place a saucepan and a small bowl next to each other on the counter. Place bag over saucepan and cut one of the corners of the Ziploc bag. Let sauce pour into saucepan until you see fat layer getting to bottom of the bag. Move bag immediately over small bowl for fat to pour out.
To thicken sauce, mix cornstarch with 4 tablespoons water. Heat sauce in pan until just boiling. Whisk in cornstarch slurry and cook a few minutes until slightly thickened.
Nutrition facts assume 2 pounds of short ribs (net of bones) and do not calculate removal of fat from sauce.