I found this recipe 25 years ago in either Bon Appetite or Gourmet magazine. I have altered it a little bit to make it creamier. I serve this every Christmas with prime rib. It has become a family tradition.
2 cups whipping cream
2 cups half and half
4 lbs. russet potatoes, peeled and thinly sliced
½ cup cream-style horseradish
1 ½ cup shredded Gruyere cheese
2 Tbsp. minced fresh rosemary (I am generous with this ingredient because I love the flavor of rosemary)
Preheat oven to 400 degrees. Lightly butter 8x8x2 glass baking dish. Bring cream and half-and-half to boil in a heavy large Dutch oven over medium-high heat. Add potatoes. Simmer until potatoes just begin to soften, about 5 – 10 minutes. Using a slotted spoon, transfer potatoes to dish, placing half of them in dish then topping with ¾ cup cheese followed by the other half of potatoes and cheese. Mix horseradish and rosemary into liquids in pot. Season generously with salt and pepper and cook until thickened. Pour over potatoes and press to compact. Cover with foil and bake for 40 minutes. Uncover and bake until potatoes are tender and top is brown, about 30 minutes. Let stand 10 minutes before serving.