Slow Cooker London Broil

Remove the London Broil from refrigeration and unpackage. Season the beef well on all sides with kosher salt and freshly ground black pepper, letting it rest, uncovered, at room temperature for one hour.
2 pounds London Broil roast, kosher salt and freshly ground black pepper
In a large, deep heavy-bottom skillet (such as cast iron) or a Dutch oven over medium-high heat, add the oil. When the oil is hot, carefully add the rested and seasoned London Broil, searing it on all sides.
2 tablespoons avocado or olive oil
Remove the seared meat from the pan to a well-greased, preheated 6-quart slow cooker. Top the London Broil with the diced onions and minced garlic.
1 medium yellow or white onion, 5 cloves garlic
In a large measuring cup combine the beef broth, tomato paste, balsamic vinegar, brown sugar and Worcestershire sauce; slowly and gently pour this over the London Broil.
1 cup beef broth, 3 tablespoons tomato paste, 1/2 cup good-quality balsamic vinegar, 2 tablespoons dark brown sugar, 1 tablespoons Worcestershire sauce
Cover the slow cooker. Cook the London Broil on low for 6-8 hours, or until the beef is very fork-tender when tested at the center with a fork and easily shreds apart. Serve at once, dressed with the balsamic sauce from the slow cooker, as desired.