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
