Melt 1/4 cup butter in a large skillet over medium heat. Add olive oil, onions, and sugar. Cook, stirring often, 25 to 30 minutes or until onions are deep golden brown. Remove from heat, and set aside.
Bring potatoes and chicken broth to a boil in a Dutch oven; cook 20 minutes or until tender. Drain. Reserve broth for another use, if desired.
Add remaining 1/2 cup butter to potatoes; mash with potato masher until smooth. Whisk together whipping cream and next 3 ingredients; add to potato mixture, stirring until blended.
Spoon 1/2 the potato mixture into a lightly greased 13- x 9-inch baking dish. Spoon caramelized onions evenly over potatoes. Spoon remaining potatoes evenly over onions.
Bake at 350° for 30 to 35 minutes or until golden.
Note: Casserole may be prepared a day ahead and refrigerated overnight. Remove from refrigerator 30 minutes before baking. Bake at 350° for 40 to 45 minutes or until golden brown.