Toss sliced potatoes into a greased 13X9 cake pan. Melt 3 Tablespoons of butter and pour over potatoes along with salt and pepper. Use hands to toss potatoes in butter, salt & pepper and spread them evenly on the bottom of the pan. Add ground beef, peas and all of the cheese saving a handful or two for the top. Set aside. In a medium sized sauce pan melt two tablespoons of butter, once the butter has melted add 3 tablespoons of flour mix until it forms a paste. Next add 3 Cups of milk whisking until the butter flour mixture dissolves into the milk. Season milk mixture with salt, pepper, and garlic powder. Bring to a boil and turn off heat, mixture will have thickened. Pour the entire mixture over the potatoes. Cover with foil and bake at 375 degrees for one hour or until potatoes are soft. Take foil off add remaining cheese and bake for 10 minutes or until cheese is melted and slightly brown.