Preheat oven to 375. Bring a large pot of salted water to the boil. Add potatoes, bring back to the boil, drain and reserve. Melt 1/2 butter in medium skillet and cook onion, stirring over medium heat until lightly browned, about 3 minutes. Remove from heat. Layer a baking dish with 1/2 of the potatoes, half of the flour, salt and pepper, 1/2 of the ham, all of the onions. Repeat ending with the ham. Heat milk until warm. In a glass measuring cup stir mustard into 1 tablespoon of the milk. Stir in 1/2 cup more milk, add remaining milk and pour over all. Dot with remaining butter. Cover and bake for 30 minutes. Uncover and sprinkle cheese on top. Bake 30 minutes more or until potatoes are tender. Recipe by: Cuisinart Magazine Posted to MC-Recipe Digest V1 #1014 by KSBAUM on Jan 15, 1998