Cheesy Mashed Potato Casserole is an ultra rich and creamy casserole with 3 types of cheese. Makes a great Thanksgiving side and also goes great with chicken, beef, or pork.
Place potatoes and a tablespoon of salt in a pot. Add water to cover by 1 inch.Bring to a boil and then reduce heat to a simmer. Cook until easily pierced with a fork, about 20 minutes. Drain and return to pot.
Preheat oven to 375 degrees and grease a 9x13-inch baking dish.
Add butter and mash until smooth.Stir in half-and-half, salt, pepper, and green onions.
Stir in cheddar cheese and mozzarella until melted.Transfer to prepared baking dish.
In a small bowl, stir together Panko topping ingredients. Sprinkle on top of casserole. Bake for 30 minutes.