This potato side dish casserole has a rich creamy sauce. Try it at the holidays instead of mashed potatoes.
Heat butter and olive oil in a large skillet over medium heat. Add onions; cook slowly until browned, stirring occasionally, about 30 minutes.
Heat oven to 350°F. In a large bowl, whisk together chicken broth, cream, flour, salt, and pepper. Stir in diced potatoes, cooked onions, and 1/2 cup cheese. Pour mixture into a 2-quart baking dish, and top with remaining 3/4 cup cheese. Cover with foil; bake 45 minutes. Remove foil, and bake 30 minutes more. Let cool; cover and refrigerate up to 3 days.
On Thanksgiving Day, remove potatoes from refrigerator and bring to room temperature. Heat oven to 375°F. Heat in microwave for 20 minutes at 50 percent power to warm, then bake in oven for 20 minutes. Broil 3 minutes to brown top.