Preheat oven to 400ºF and grease a 9x13 inch baking dish.

Place half of the sliced potatoes in the baking dish and top with salt and pepper.

In a medium saucepan, heat butter until completely melted. Add flour, salt, and onion powder and stir constantly for one minute.

Add milk and bring to a boil, stirring constantly. Reduce to a simmer and continue stirring until mixture thickens, about ten minutes.

Stir in all of the cheese at once and continue stirring until cheese is completely melted.

Pour half the cheese over the half of potatoes already in dish. Add the rest of the potatoes, salt and pepper to taste, and then top with the rest of the cheese sauce.

Top with the Panko bread crumbs.

Cover with aluminum foil and bake for 1 & 1/2 hours. {You can remove the foil during the last ten minutes or so, if you want to brown up the bread crumbs a bit.}