grease a standard 12-cup muffin tin with butter or a bit of neutral oil, making sure to coat the sides and bottoms so the potatoes release easily after baking.
Peel the potatoes and slice them as thinly as you can, about 1/8 inch thick or thinner. A mandoline makes this quick and even, but a sharp knife and a bit of patience work just as well. Keep the slices in a bowl as you work.
In a small bowl or measuring cup, stir together the heavy cream, minced onion, salt, and black pepper. This seasoned cream is what gives the potatoes that classic, old-fashioned au gratin flavor with only a few ingredients.
Sprinkle about half of the shredded cheddar (1 cup) evenly into the bottoms of the 12 muffin cups, dividing it as evenly as you can. This cheese will melt and help form a browned, lacy edge around
