Heat olive oil in a large skillet over medium-high heat.
Add beef sirloin tips in batches and brown on all sides, about 3-4 minutes per batch. Transfer browned beef to the slow cooker.
In the same skillet, add diced onion and cook for 3-4 minutes until softened.
Add minced garlic and cook for an additional 1 minute until fragrant.
Sprinkle flour over the onions and garlic, stirring constantly for 1-2 minutes to cook the flour.
Gradually whisk in beef broth and water, scraping up any browned bits from the skillet.
Stir in Worcestershire sauce, dried thyme, salt, and black pepper.
Pour the mixture over the beef in the slow cooker.
Add sliced mushrooms to the slow cooker and stir gently to combine.
Cover and cook on low for 6 hours, or until beef is tender.
About 15 minutes before serving, mix cornstarch and cold water in a small bowl until smooth.
Stir the cornstarch slurry into the slow cooker to thicken the gravy.
Cover and cook on high for 15 minutes until the gravy has thickened.
Serve hot over mashed potatoes, rice, or egg noodles.