Prepare the vegetables: Remove any tough or damaged outer leaves from the cabbage. Cut the cabbage into 6–8 thick wedges, keeping the core attached so the wedges hold together. Scrub the potatoes well (peel if you prefer) and cut them into large chunks, about 1 1/2 inches. Peel the onion and slice it into thick rings or half-moons.
Layer the slow cooker: Place the potato chunks in an even layer on the bottom of a 5- to 7-quart slow cooker. Scatter the sliced onion over the potatoes. Arrange the cabbage wedges on top, nestling them in so they fit snugly but stay mostly intact.
Season and add butter: Sprinkle the salt and pepper evenly over the vegetables. Dot the top with the butter pieces, tucking some down between the cabbage wedges and potatoes so it melts throughout the dish.
