Preheat the oven to 350. Fill a 6 quart pan with water. Peel the potatoes and put the potatoes in the water whole. Boil until a fork pierces the potato flesh easily. Remove the potatoes from the water with tongs. Set on a cutting board and cool until you are able to handle without burning yourself. Hot potato! Once cooled enough to touch, cut into ¼ - ½ inch slices (close is good enough) and place in a 3 quart casserole dish.

In a medium sauce pan while the potatoes are cooling, add the butter and allow to melt over medium heat. Add the flour and allow it to cook with the butter for 1-2 minutes. Add the garlic and cook for another 2 minutes. Use a whisk and whisk in the milk. Allow to cook - stirring constantly until milk is warm. Add the salt and the cayenne. Stir in the cheddar and the Parmesan and cook till melted. Pour the cream/cheese mix over the potatoes and using a fork move the potatoes around to allow the cream to go between the potato pieces.

Bake at 350 for 45 minutes. Let stand for 5 minutes before serving. Garnish with pepper and green onion.