Prepare the Beef:
Season beef with salt and pepper.
Place in slow cooker along with sliced onions, minced garlic, beef broth, Worcestershire sauce, and thyme.
Cook:
Cover and cook on low for 7–8 hours or high for 4–5 hours, until beef is tender and shreds easily.
Make the Gravy:
Remove beef from the slow cooker and set aside.
Mix cornstarch and water in a small bowl to form a slurry.
Whisk slurry into the hot cooking liquid and cook on high for 5–10 minutes, or until thickened.
Make Mashed Potatoes:
Boil chopped potatoes in salted water until fork-tender.
Drain, mash, and mix in butter, milk or cream, and salt to taste.
Assemble the Dish:
Plate a scoop of mashed potatoes.
Top with shredded beef and a generous ladle of gravy.
Serve hot and enjoy!