In a medium bowl, whisk together the soy sauce, balsamic vinegar, Worcestershire sauce, ¾ cup of the beef broth, brown sugar, dried rosemary, dried thyme, black pepper, and olive oil to create the marinade.

Place the London broil in a resealable plastic bag or shallow dish and pour the marinade over the meat. Ensure the meat is evenly coated with the marinade. Seal the bag or cover the dish, then refrigerate for at least 4 hours, or preferably overnight, to allow the flavors to infuse the meat.

After marinating, heat a skillet over medium-high heat. Sear the London broil on all sides for 2-3 minutes per side to lock in the flavors. Once seared, transfer the meat to the slow cooker.

Arrange the sliced onion, carrot chunks, potato chunks, and garlic cloves around the London broil in the slow cooker.

Pour the remaining 1 cup of beef broth over the meat and vegetables. The liquid should come about halfway up the side of the meat.

Cover and cook on low for 6-8 hours, or until the meat is fork-tender.

Once the cooking time is complete, carefully remove the London broil from the slow cooker and let it rest for 10-15 minutes before slicing against the grain.

If desired, strain the liquid from the slow cooker into a saucepan. In a small bowl, mix the cornstarch with a few tablespoons of cold water to create a slurry. Add the slurry to the strained liquid and bring to a boil, stirring constantly, until the sauce thickens.

Serve the sliced London broil alongside the cooked vegetables and drizzle with the savory sauce. Enjoy!