Season the steak generously with salt and pepper on both sides.
In a large skillet, heat olive oil over medium-high heat. Brown the steaks on both sides, about 3-4 minutes per side. Transfer the steaks to the slow cooker.
In the same skillet, add sliced onions and minced garlic. Cook until onions are caramelized, about 8-10 minutes.
Transfer the onions and garlic to the slow cooker, arranging them over the steaks.
In a mixing bowl, whisk together beef broth, Worcestershire sauce, balsamic vinegar, tomato paste, dried thyme, and dried rosemary. Pour the mixture over the steaks and onions in the slow cooker.
Cover and cook on low for 6-8 hours, or until the steak is tender and easily shreds with a fork.
If desired, you can thicken the sauce before serving.
To do this, remove about 1/4 cup of the cooking liquid from the slow cooker and whisk it with cornstarch until smooth.
Stir the mixture back into the slow cooker and let it cook for an additional 10-15 minutes until thickened.
Serve the braised steak and onions hot, garnished with fresh herbs if desired.
Enjoy with mashed potatoes, rice, or crusty bread.