Peel and thinly slice potatoes.
Bring potatoes, 1 teaspoon salt, and water to cover to a boil in a Dutch oven; cook 8 to 10 minutes. Remove from heat; drain and set aside.
Melt butter in a large skillet over medium-high heat; add chopped onion, and sauté 12 to 15 minutes or until golden brown.
Layer half of potatoes in a lightly greased 13- x 9-inch baking dish or 2 (8-inch) square baking dishes; sprinkle with 1/2 teaspoon salt and 1/4 teaspoon pepper. Top with half each of onions, Gruyère cheese, and Cream Sauce. Repeat layers once, ending with Cream Sauce.
Bake at 350° for 1 hour and 15 minutes or until golden brown.
*1 (8-ounce) block Swiss cheese, shredded, or 1 (8-ounce) block sharp white Cheddar cheese, shredded, may be substituted.