Potato Casserole with Caramelized Onions

Ingredients (10)

• 3/4 cup (1½ sticks) butter,
• 1/4 cup olive oil
• 6 large sweet onions,
• 1 tablespoon granulated white sugar
• 4 pounds baking potatoes,
• 4 cups chicken broth
• 1 cup whipping cream
• 1 cup sour cream
• 4 large eggs,
• 1/2 teaspoon salt