In a small bowl, whisk together the minced garlic, beef broth, soy sauce, butter, Worcestershire sauce, black pepper, and red pepper flakes (if using).
Pour the liquid mixture over the beef in the slow cooker.
Cover and cook on low for 3-4 hours, or until the beef is tender and cooked through.
Serve hot, perhaps with rice or mashed potatoes.
Notes
Adjust cooking time based on your slow cooker's performance. The sauce will thicken slightly as it cooks.