Casserole Kentang Tumbuk dengan Krim Asam dan Daun Bawang

Bahan-bahan (9 porsi)

• 6 pounds Yukon gold potatoes (peeled and cut into chunks)
• 1 cup unsalted butter, (2 sticks, softened and divided)
• 1 teaspoon teaspoon
• 1 ½ cups sour cream
• 1 teaspoon black pepper
• 3 tablespoons finely chopped chives
• ⅔ cup bread crumbs
• ⅔ cup grated Parmigiano-Reggiano cheese