In a large skillet over high heat, sear the beef so that outside edges are brown. Add butter and diced onion to saute while searing the beef. Continually stir onions to prevent sticking. Transfer both onions and beef to your crockpot once fully seared.
Set the crockpot on high and add mixed vegetables, cubed potatoes (I leave the skin on like my grandma did for extra vitamins and flavor but you can peel them if you prefer), diced tomatoes, beef broth, beef bullion, Worcestershire sauce, salt, pepper, and bay leaves.
Let cook for 1 hour on high (or 2 hours on low). Check potatoes to be sure they are fully cooked through then serve.