Melt 1/4 cup (1/2 stick) of the butter in a large skillet over medium heat. Add the olive oil, onions and sugar. Cook, stirring often, 25 to 30 minutes or until the onions are deep golden brown. Remove from heat, and set aside.
Preheat the oven to 350 degrees F. Spray a 9x13-inch casserole dish with nonstick spray.
Bring the potatoes and chicken broth to a boil in a Dutch oven; cook 20 minutes or until tender. Drain. Reserve the broth for another use, if desired.
Add the remaining 1/2 cup (1 stick) of butter to the potatoes; mash with a potato masher until smooth. In a medium bowl, whisk together the whipping cream and remaining ingredients; add to the potato mixture, stirring until blended.
Spoon 1/2 of the potato mixture into the prepared baking dish. Spoon the caramelized onions evenly over the potatoes. Spoon the remaining potatoes evenly over the onions.
Bake for 30 to 35 minutes or until golden.
Notes
*MAKE AHEAD: The casserole may be prepared a day ahead and refrigerated overnight. Remove from refrigerator 30 minutes before baking. Bake at 350 degrees F. for 40 to 45 minutes or until golden brown.
*If you are preparing this recipe as gluten-free, just be sure to use a brand of chicken broth that is GF.