4 lbs. Yukon gold potatoes, peeled and cut into 1/8-in. slices
Preheat oven to 375°. Lightly spray 13 x 9-inch baking dish with no-stick cooking spray; set aside.
Melt margarine in 6-quart stockpot over medium heat and cook onion, stirring occasionally, until tender, about 3 minutes.
Stir in flour with wire whisk and cook 1 minute. Slowly whisk in milk, heavy cream and pepper. Add sage and bring to a boil, stirring frequently, 1 minute.
Remove from heat, then remove sage. Whisk in Knorr® Homestyle Stock and 1-1/2 cups cheese; set aside. (Sauce will be very thick).
Toss potatoes with cheese sauce, then evenly spread in prepared baking dish. Bake covered 50 minutes. Remove cover, then sprinkle with remaining 1/2 cup cheese. Bake uncovered an additional 20 minutes or until golden.