In a skillet heat the olive oil over low to medium heat, add the onions and cook until the onions are almost caramelized. Use up to 2 tablespoons of water to help in the cooking process if onions start to stick to the pan. This takes about 30-35 minutes.
In a slow cooker place the beef roast, caramelized onion, and garlic.
Add the balsamic vinegar, beef broth, worcestershire sauce, salt, pepper and fresh thyme.
Cook on high for 4-6 hours or on low for 8-10 hours.
Note: the onions can be caramelized the day before and stored in the refrigerator until ready to add to the slow cooker.