These potatoes are actually quite simple to make, requiring few ingredients, but they look lovely in a serving dish because of the accordion-style cuts.
Use a 6-quart slow cooker. Wash the potatoes and pat dry, but do not peel them. Carefully slice each potato crosswise every ½ inch or so, but do not cut all the way through, making about 6 cuts per potato. The potatoes should start to separate a bit, similar to an accordion. In a small bowl, whisk together the garlic, oil, butter, rosemary, salt, and pepper. Use a pastry brush to brush each potato with the mixture, taking care to brush in between the slices.
Place the potatoes into the slow cooker insert. Cover and cook on low for 6-8 hours or on high for about 4 hours. The potatoes are finished when a fork inserts easily and the potato pulp is fluffy.