Fry the bacon in a heavy cast-iron skillet to desired doneness and drain on paper towels. Heat the bacon drippings over medium heat, add the potatoes and onions and sprinkle with salt and pepper. Fry until the potatoes are browned and crisp on the outside and tender on the inside and the onions are tender. (Do not turn the potatoes until they are crisp.) Taste for seasoning.
Grill or broil the bread until toasted on one side. Smear the untoasted sides generously with mustard and layer the bacon, potatoes and onions on top of one mustard side. Cover with the remaining bread, mustard side down.
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