Peel the potatoes and slice them into thin rounds, about 1/8 inch thick, using a sharp knife or mandoline. Place the slices in a large bowl and keep them loosely covered with a clean kitchen towel while you prep the onions so they don’t dry out.
Peel the onions and slice them into thin half-moons. Try to keep the slices roughly the same thickness as the potatoes so everything cooks evenly and softens at the same time.
In a small bowl, stir together the flour, salt, and pepper (if using). This simple mixture will lightly coat the layers and help turn the butter and onion juices into a silky, almost gravy-like coating as everything cooks.
Arrange a single, even layer of potato slices on the bottom of the slow cooker, overlapping slightly like shingles. Scatter a layer of onions over the potatoes. Sprinkle a teaspoon or so of the flour mixture evenly over the onions.
Dot a few small pieces of butter (about 1 tablespoon total) over the top of the flour and onions. This helps the top of each layer get that buttery, golden finish as it cooks.
Repeat the layers—potatoes, onions, a light sprinkle of the flour mixture, and small dots of butter—until you’ve used all of the potatoes and onions. Try to finish with a potato layer on top, then dot the remaining butter all over the surface. This top layer is what will get those crispy browned spots and golden edges.
Cover the slow cooker with the lid and cook on HIGH for 3 1/2 to 4 hours or on LOW for 6 to 7 hours, until the potatoes are very tender when pierced with a fork and the onions are soft and lightly caramelized. Avoid lifting the lid during the first few hours so the heat and steam stay trapped inside.
Once the potatoes are tender, remove the lid and, if your slow cooker allows, let the dish sit on WARM or LOW for another 15 to 20 minutes with the lid slightly ajar. This helps some of the excess moisture evaporate and encourages more golden, crispy spots on the top layer.
Serve the potato and onion bake directly from the slow cooker, scooping down through the layers so everyone gets some of the buttery, caramelized onions and the tender potatoes with browned edges. Taste and add a pinch more salt at the table if needed.
