Prepare the beef. Pat the beef tips or stew meat dry with paper towels. Season with salt, black pepper, garlic powder, and onion powder.
Sear the beef. Heat the olive oil in a large skillet over medium-high heat. Add the beef in batches and sear for 2–3 minutes per side, until browned. The beef does not need to be cooked through. Transfer the browned beef to the crockpot.
Add the aromatics. Add the sliced onion and minced garlic to the crockpot. If using mushrooms, add the sliced mushrooms as well.
Make the gravy mixture. In a bowl, whisk together the beef broth, Worcestershire sauce, and cream of mushroom soup until reasonably smooth.
Add the gravy. Pour the gravy mixture over the beef and vegetables in the crockpot. Stir gently to combine.
Slow cook. Cover and cook on LOW for 6–8 hours or HIGH for 3–4 hours, or until the beef is fork-tender. Cooking time can vary depending on the size of the beef pieces and the slow cooker.
Thicken the gravy. About 15–20 minutes before serving, combine the cornstarch and cold water in a small bowl. Whisk until smooth. Stir the slurry into the gravy in the crockpot.
Finish cooking. Replace the lid and continue cooking for approximately 15–20 minutes, or until the gravy has thickened.
Taste and adjust. Taste the gravy and add additional salt or black pepper if needed.
Serve. Spoon the tender beef tips and gravy over mashed potatoes, egg noodles, or rice. Garnish with fresh parsley if desired.
