This cheesy, creamy potato gratin will definitely be a hit at Thanksgiving!
Preheat oven to 350 degrees F. Grease the inside of an 8x8 baking dish with a small amount of the butter.
Combine the potatoes, onion, half and half, and butter into a large sauce pot. Bring up to a simmer, and cook for 5 minutes. The potatoes should not be fully cooked; this step is used to thicken the half and half.
Strain the potatoes and place the half and half mixture into a separate sauce pan. Whisk in the nutmeg, garlic, and thyme. Bring to a simmer and whisk in the cheese. Season with salt and pepper.
Place the potatoes in a layer at the bottom of the baking dish, followed by a light seasoning of salt and pepper. Pour some of the gruyere mixture over the potatoes, and repeat until all of the potatoes and liquid have been used. The potatoes should not be drowning, but rather moist with the liquid coming up the sides by about 3/4-incb,
Top the last layer with a sprinkling of breadcrumbs and Parmigiano.
Bake the potatoes until the outsides are bubbly, the top is caramelized, and the potatoes are cooked, about 1 hour. Garnish with the remaining thyme leaves.
Allow to cool for 20 minutes before serving.