Peel the russet potatoes and cut them into rustic chunks about 1 to 1 1/2 inches in size. You want them big enough to hold their shape but small enough to get tender all the way through.
Scatter the sliced onions evenly over the bottom of a large slow cooker. This creates a soft, sweet base as they cook down and release their juices.
Layer the potato chunks on top of the onions in an even layer, without packing them too tightly. This lets the broth move around and cook everything evenly.
Pour the beef broth over the onions and potatoes, making sure the vegetables are mostly submerged. Gently press down with the back of a spoon to settle everything into the liquid, but do not stir too vigorously.
Cover the slow cooker with the lid and cook on LOW for 8 to 10 hours, or on HIGH for 4 to 5 hours, until the onions are very soft and golden and the potatoes are tender when pierced with a fork.
Once the cooking time is up, gently stir the soup from the bottom to lift the soft, caramelized onions through the broth. Some of the potatoes will break up a bit, naturally thickening the soup and giving it that slightly creamy, hearty texture.
Taste the broth and, if desired, adjust the seasoning at the table with a pinch of salt or black pepper, but keep in mind that the whole spirit of this dish is its simplicity. Ladle the hot soup into bowls, making sure each serving gets plenty of both onions and potato chunks along with the rich golden broth, and serve steaming from the slow cooker.
