Spray slow cooker with non-stick cooking spray.
In a medium bowl, mix together flour, salt, garlic powder, and pepper. Dredge meat tips in the flour mixture.
Heat olive oil in large skillet over medium-high heat, then brown tips for a couple of minutes (you aren't cooking them all the way through, just browning the edges). Place beef tips in slow cooker.
Dump onions and mushrooms on top of meat in slow cooker.
Mix broth, Worcestershire, ketchup, red wine vinegar, water in a bowl. Pour on top of meat in slow cooker.
Sprinkle Italian seasoning on top of everything and cook on low for 7-8 hours.
After meat is finished cooking, mix cornstarch together with water in a separate bowl. Using a strainer, remove beef tips from slow cooker and place in a separate bowl. Pour cornstarch mixture into remaining liquid in the crock pot and mix well. Add the beef tips back into the slow cooker and cook on high for about 10 minutes or until the liquid starts to thicken and turns into a gravy.
Serve over hot mashed potatoes.