Heat the oven to 375°F and grease a 9-by-13-inch dish. Melt 3 tablespoons butter, stir in flour for 1 minute, then whisk in milk, mustard, salt and pepper until lightly thickened.
Remove from heat and stir in the cheddar a handful at a time until smooth.
Layer half the potatoes and sauce in the dish, then repeat. Cover tightly with foil.
Bake for 55 minutes. Mix crackers with the remaining melted butter, uncover the pan and sprinkle them evenly over the top.
Bake 15 to 20 minutes more until a knife slides through the center and crumbs are golden. Rest 15 minutes.