Heat the oil in a large skillet or the insert of a slow cooker (if your cooker has a browning feature) over medium-high heat, and sear the round steak, 3-4 minutes per side, or until both sides are nicely browned. (I like to add the onions and bell pepper to the skillet or cooker around the meat, after it has been flipped, to sear them a little bit, too.)
If using a separate skillet, transfer the meat mixture to the slow cooker. Add the tomato sauce, and then fill the can halfway with water**, swirl it around to get all the sauce out, and add to the meat. Then add the onions and bell pepper (if you didn’t sear them with the meat), garlic powder, Worcestershire sauce, salt and pepper. (I use 3/4 teaspoon kosher salt and about 1/2 teaspoon pepper.)
Cook for 4 hours on high or 6-7 hours on low. Test with fork and if meat breaks apart easily, it’s done. For a delicious sauce, use an immersion blender to incorporate the onion and green pepper into the tomato sauce gravy, which thickens it nicely. As an option, the mixture could be transfer to a blender.