Add the remaining 1 tablespoon oil and onion to the pan; cook, stirring occasionally, until starting to brown, 3 to 5 minutes. Stir in tomato paste, garlic and thyme; cook, stirring, for 1 minute. Add vinegar and cook, scraping up any browned bits, until the liquid is mostly reduced, 3 to 5 minutes. Transfer to the slow cooker and add broth. Cover and cook on High for 4 hours or Low for 8 hours.