Special Equipment Suggested

A food processor (useful for slicing and grating); an 8-quart covered kettle; a heavy-bottomed 3-quart saucepan for the sauce; a buttered 6-quart baking dish about 3 inches deep for the final dish.

The Potatoes

Have ready the kettle containing 4 cups salted cold water to cover. Peel the potatoes and slice them 1/4 inch thick, dropping them into the cold water as you do so. Cover the kettle, and bring to the boil. Uncover and boil slowly 3 - 4 minutes until barely cooked through--eat several slices to check. Drain, cover the kettle again for 3 - 4 minutes to firm up the potato slices, then uncover.

The Garlic and Mustard Sauce (about 8 cups)

Cook the butter and flour together in a 3-quart saucepan over moderate heat until they foam and froth for 2 minutes without coloring. Remove from heat. Pour in half of the hot milk, whisking vigorously to blend thoroughly as you pour in the rest. Simmer 3 minutes, stirring. Remove from heat and whisk in the seasonings; bring to a simmer again, taste carefully, and add additional seasonings to taste.

To Assemble

Spoon a 1/16-inch layer of sauce on the bottom of the buttered baking dish. Set aside 3 cups of sauce and 1 cup of cheese for the topping. Now, to arrange everything in 4 layers, start with a quarter of the potatoes, then a quarter of the ham, followed by a third of the sauce, and sprinkle on a third of the cheese. Continue in layers, finishing the final layer with the remaining potatoes and ham. Spread on the reserved 3 cups of sauce to cover completely, then the last cup of cheese.

This may be prepared a day in advance--cover when cool and refrigerate. The final baking will be about 45 minutes at 375°. An hour or so before you plan to serve, preheat the oven to 375°. Bake in the upper third level just until the potatoes are bubbling hot and the top has browned nicely. This may be kept warm uncovered on a hot tray, but be sure not to overheat or the potatoes and ham will dry out.