Preheat oven to 350 degrees F. Cut potatoes into 1/4-inch thick slices and set aside.
In a medium saucepan over medium heat, melt butter. Add onion and cook for 5 minutes, stirring occasionally. Whisk in flour and cook for 1 minute. Whisk in milk, dry mustard, cayenne pepper and salt. Bring to a boil over high heat, stirring occasionally. Reduce heat and simmer for 3 minutes or until thickened, stirring frequently.
Arrange half of potato slices in a lightly greased 9-inch square glass baking dish or 1-1/2 qt. casserole dish. Spread half of milk mixture over potatoes and top with 1 cup cheese. Repeat layering with remaining potatoes, milk mixture and 1 cup cheese.
Bake for 1 hour, or until sauce is bubbly and potatoes are tender. Sprinkle with fresh parsley and a little cayenne pepper or paprika.
For this recipe and more, visit www.thecomfortofcooking.com