In a medium bowl, whisk together the condensed cream of mushroom soup, beef broth, and dry onion soup mix until mostly smooth. This mixture will become the savory gravy for the stew.
Pour the soup mixture evenly over the meatballs and potatoes, gently nudging with a spoon so most of the meatballs and potatoes are coated. Do not stir vigorously; you want the potatoes to stay in a layer on the bottom.
Cover the slow cooker with the lid and cook on LOW for 6 to 8 hours, or on HIGH for 3 to 4 hours, until the potatoes are very tender when pierced with a fork and the meatballs are heated through.
Once cooked, gently stir from the bottom to coat the potatoes and meatballs in the thickened sauce. If the stew seems too thick, you can loosen it with a splash of additional warm beef broth; if it
