Place the beef sirloin tips in the slow cooker. Add the diced onion, sliced mushrooms, and minced garlic on top of and around the beef.
In a bowl, whisk together the cream of mushroom soup, onion soup mix, beef broth, worcestershire sauce, and black pepper until combined. Pour the mixture over the beef and vegetables.
Cover and cook on low for 7 to 8 hours or on high for 4 to 5 hours, until the beef tips are extremely tender.
In a small bowl, whisk the cornstarch and cold water together until smooth. Stir this slurry into the slow cooker.
Turn the heat to high if not already, and let the gravy thicken for about 15 to 20 minutes with the lid slightly cracked, stirring once halfway through.
Taste the gravy and adjust seasoning with additional salt and pepper if needed. The gravy should be rich, glossy, and perfectly thick.
Serve the beef tips and gravy over egg noodles, mashed potatoes, or rice. Garnish with chopped fresh parsley.