In a medium bowl, whisk together the condensed cream of mushroom soup, beef broth, and onion soup mix until mostly smooth. It doesn’t have to be perfect, just well combined so the flavors are mixed.
Pour the soup mixture evenly over the potatoes in the slow cooker, nudging it around with a spoon so most of the potatoes are coated or at least touched by the liquid.
With clean hands, take the frozen meatballs straight from the bag and dump them in an even layer over the top of the sauced potatoes. No need to thaw or brown them first; just spread them out so they’re mostly in a single layer.
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 and plump.
Once cooked, gently stir the stew from the bottom, pulling some of the potatoes and gravy up over the meatballs. The potatoes will thicken the sauce slightly as you stir, giving you a rich, stew-like gravy.
Taste and adjust seasoning if needed. The onion soup mix is usually salty enough, but you can add a little black pepper or a pinch of salt if your broth is low-sodium.
Let the stew sit with the lid off for about 5 minutes to thicken slightly, then ladle into warm bowls and serve hot.
