Pat the beef cubes dry with paper towels and season lightly with salt and pepper. Place the beef in your slow cooker.
Scatter the sliced onion and minced garlic over the beef.
In a separate bowl, whisk together the condensed cream of mushroom soup, beef broth, Worcestershire sauce, Dijon mustard, dried thyme, black pepper, and salt. Pour this mixture evenly over the beef and onions.
Cover the slow cooker and cook on low for 6-8 hours, or on high for 3-4 hours, until the beef is tender and easily shredded with a fork.
About 30 minutes before serving, if you prefer a thicker gravy, whisk together the cornstarch and cold water in a small bowl until smooth. Stir this slurry into the slow cooker. Continue to cook, uncovered, on high for the remaining time, allowing the gravy to thicken.