Melt 1/4 cup butter in a large skillet over medium heat.

Add olive oil, diced sweet onions, and sugar to the skillet.

Cook, stirring often, for 25 to 30 minutes or until onions are deep golden brown.

Remove from heat and set caramelized onions aside.

Bring peeled and cubed potatoes and chicken broth to a boil in a Dutch oven.

Cook potatoes for 20 minutes or until tender.

Drain the potatoes, reserving broth for another use if desired.

Add remaining 1/2 cup butter to potatoes.

Mash potatoes with a potato masher until smooth.

In a separate bowl, whisk together whipping cream, sour cream, eggs, and salt.

Add the cream mixture to the mashed potato mixture, stirring until blended.

Spoon 1/2 of the potato mixture into a lightly greased 13- x 9-inch baking dish.

Spoon the caramelized onions evenly over the potatoes.

Spoon the remaining potato mixture evenly over the onions.

Bake at 350°F (175°C) for 30 to 35 minutes or until golden.

If preparing ahead and refrigerating, remove from refrigerator 30 minutes before baking.

Bake refrigerated casserole at 350°F (175°C) for 40 to 45 minutes or until golden brown.