Season steaks with salt and pepper. Add to slow cooker. Slice onions and add to slow cooker. Then, add garlic powder, onion soup mix, and beef broth.
Set slow cooker to low for 8 hours.
One hour before you’re ready to serve, mix cold water and cornstarch in a small bowl. Stir into slow cooker. Cook on high until gravy has thickened, stirring periodically. Serve over buttered egg noodles or rice.