Lay the sliced onions in an even layer on the bottom of a 5- to 6-quart slow cooker. This gives the beef a soft bed to rest on and helps flavor the gravy.
Place the raw beef stew meat on top of the onions, spreading it out so it cooks evenly.
In a medium bowl, whisk together the condensed cream of mushroom soup, dry onion soup mix, and beef broth (or water) until fairly smooth.
Pour the soup mixture evenly over the beef and onions, making sure everything is lightly coated. Do not stir; the layers will settle as it cooks.
Cover the slow cooker with the lid and cook on LOW for 7 to 8 hours, or on HIGH for about 4 hours, until the beef is very tender and the gravy is rich and bubbling.
Once done, give everything a gentle stir to combine the onions, beef, and gravy. Taste and adjust seasoning with a little salt and black pepper if needed.
Serve the beef and gravy hot over mashed potatoes, egg noodles, or rice, spooning extra gravy over the top.