COLLARD POTATO SALAD WITH MUSTARD DRESSING

2 pounds small red potatoes, scrubbed
1 pound collards, coarse stems discarded and the leaves washed well and cut into 1-inch pieces
2 tablespoons Dijon-style mustard
2 tablespoons red-wine vinegar
1/3 cup olive oil
6 slices of lean bacon, cooked until crisp, drained, and crumbled (optional)
3 tablespoons thinly sliced scallion 

In a kettle 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. 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. Drain the collards in a sieve, refresh them under cold water, and squeeze them dry in a kitchen towel. 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. Add the collards, pulling them apart to separate the leaves, the bacon, and the scallion and toss the salad well.