Prepare the Base:

In the same pan, sauté onions and garlic until soft. Stir in tomato paste and cook for 1 minute.
Deglaze with red wine (if using) and simmer for 3 minutes.
Combine Ingredients in the Slow Cooker:

Place the seared roast into the slow cooker. Add diced tomatoes, beef broth, carrots, celery, thyme, and rosemary. Pour the wine mixture over it.
Cook Low and Slow:

Cover and cook on LOW for 8 hours (or HIGH for 4-5 hours) until the beef is fork-tender.