In a large, heavy-duty skillet on high heat add the olive oil and half of the cubed top round beef. Sear the beef on all sides for 2 to 3 minutes or just until browned. Remove from the skillet and place on a plate. Add the other half of the cubed beef to the hot skillet and sear the pieces, all on sides, for another 2 to 3 minutes. At this point, you are just looking to get a good sear on the beef cubes and not cook them all the way through. You will still have some pink to the meat.
To the insert of your slow cooker, add the diced onions, sliced white mushrooms, and the browned top round beef cubes and their juices from the plate. There is a lot of great flavor in the juices from the seared beef.
In a large bowl, add the beef broth, packet of onion mushroom dry soup mix, Worcestershire sauce, bay leaf, salt, black pepper, and fresh chopped parsley. Stir to combine. Add the liquid mixture to the slow cooker on top of the beef and vegetables.
Set your slow cooker to HIGH heat for 3½ hours. Put the lid on your slow cooker and allow it to cook undisturbed.
After your slow cooker beef tips have cooked for 3½ hours, you will need to remove the bay leaf and make your cornstarch slurry to thicken the liquids and create a gravy.
To a small bowl, add the room temperature water and cornstarch. Stir until the cornstarch has dissolved and you have created a slurry. Remove the lid to the slow cooker and stir in the slurry to the beef broth. Allow your beef tips to continue to cook for an additional 15 to 30 minutes or until your liquids have thickened into a gravy.