Preheat your oven to 350°F (175°C).

Peel and thinly slice 3 cups of russet potatoes into uniform rounds, about 1/8 inch thick.

In a large mixing bowl, combine 1 cup of sour cream, 1 cup of shredded sharp cheddar cheese, 1/2 cup of grated Parmesan cheese, and 1/4 cup of melted butter. Stir in 2-3 diced green onions, seasoning with salt and pepper to taste.

Grease a 9x13-inch baking dish with butter or cooking spray. Start by layering half of the sliced potatoes evenly in the dish. Spoon half of the cheese mixture over the first layer of potatoes, and repeat this process with the remaining potatoes and cheese mixture, finishing with a layer of just potatoes.

Sprinkle 1 cup of breadcrumbs evenly over the top of your final potato layer.

Cover the baking dish with aluminum foil and bake for 30 minutes.

After 30 minutes, carefully remove the foil and continue baking for an additional 15-20 minutes, until golden brown and bubbly.

Remove the dish from the oven and let it cool for about 10 minutes before serving.