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In a pot combine the potatoes with enough water to cover them by 2 inches, bring the water to a boil, and simmer the potatoes 15 to 20 minutes, or until they are tender.
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Transfer the potatoes with a slotted spoon to a colander, reserving the cooking liquid, and in the reserved cooking liquid boil the collards, stirring occasionally, for 10 minutes.
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Drain the collards in a sieve, refresh them under cold water, and squeeze them dry in a kitchen towel.
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In a bowl whisk together the mustard, the vinegar, and salt and pepper to taste, add the oil in a stream, whisking, and whisk the dressing until it emulsified. Quarter the potatoes and add them to the dressing.
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Add the collards, pulling them apart to separate the leaves, the bacon (or apple), and the red onion and toss the salad well.