Also known as funeral potatoes, this casserole is to die for!
Melt the butter in a medium saucepan over medium heat. Add the mushrooms and onions and cook until tender, about 8 minutes. Season with the salt and pepper (see note).
Meanwhile, whisk together the cream, chicken broth, and cornstarch. Add to the cooked mushrooms and onions. Bring to a boil and boil for 1 minute. The mixture will thicken and emulsify.
Set aside to cool for a few minutes. Stir in the French onion dip and green onions.
Preheat oven to 375° F (190° C).
Pour 4 tablespoons of the melted butter in the bottom of a 13x9 inch baking dish.
Place 1 pound of frozen hashbrowns on top of the melted butter. Spread half of the soup base on top of the hashbrowns, then half of the shredded cheese on top of that.
Place the remaining 1 pound of hashbrowns on top of the cheese, followed by the remaining soup base and shredded cheese.
Top the casserole with the crushed cornflakes. Drizzle the remaining 2 tablespoons of melted butter on top of the cornflakes.
Bake in preheated oven for 1 hour and 10 minutes, or until the potatoes are cooked through and hot and the cornflakes are golden and crispy.
Allow to cool in the pan for 10 minutes before serving.