Preheat oven to 400°F. Butter 13 x 9 x 2-inch glass baking dish. Mix first 7 ingredients in large pot. Bring to simmer over medium-high heat. Add half of cheese; whisk until smooth. Chill remaining cheese. Add potatoes to pot; bring to simmer. Transfer potato mixture to prepared dish, spreading evenly. Cover with foil; bake 15 minutes. Uncover and bake until potatoes are very tender and liquid bubbles thickly, about 50 minutes. Dot potatoes with remaining cheese. Bake until cheese softens, about 5 minutes. Let cool 15 minutes before serving.