Season the flank steak with black pepper, paprika, thyme, and rosemary. Rub the seasonings evenly on both sides of the steak.
In a skillet, heat a small amount of oil over medium heat. Add the onion and garlic, sautéing for 2-3 minutes until softened and fragrant.
Transfer the sautéed onions and garlic to the slow cooker. Add the beef broth, soy sauce, Worcestershire sauce, balsamic vinegar, and brown sugar. Stir to combine.
Place the seasoned flank steak into the slow cooker. Cover and cook on low for 6-8 hours or until the steak is tender and easy to shred.
If you prefer a thicker sauce, remove the steak and set it aside. Mix the cornstarch with a little water to form a slurry, then stir it into the slow cooker liquid. Cook for another 10-15 minutes until the sauce thickens.
Slice the flank steak against the grain and serve with the sauce from the slow cooker. It pairs wonderfully with mashed potatoes, rice, or roasted vegetables.