Put the potatoes in a small saucepan with enough cold water to cover and season with salt. Bring to a boil and then simmer, uncovered, until just tender, about 5 minutes. Drain and put in a large bowl.
Bring a medium pot of water to a boil and salt it generously. Fill a medium bowl with ice water and salt it as well. Add the corn, beans, and broccoli to the boiling water and cook until crisp-tender, about 2 minutes. Use a slotted spoon or strainer to scoop out the vegetables and plunge them immediately into the ice water. Drain the vegetables, pat them dry, and add to the bowl of potatoes along with the tomatoes and cucumber. Toss the salad with 1/2 cup of the dressing. Taste, and add more salt and pepper, if desired. (The salad may be prepared up to this point 2 hours ahead and refrigerated.)
When ready to serve, toss the salad with the greens and the sprouts, if desired, and with a bit more dressing if you like your salad on the well-dressed side. Pass the remaining dressing at the table.
Know-How: Immersing vegetables in boiling salted water and then plunging them into ice water is called blanching and refreshing. Having the water at a full rolling boil before adding the vegetables, as well as cooking uncovered, is the key to keeping green vegetables vibrant.
1 clove garlic
1/2 teaspoon kosher salt
1 cup mayonnaise
1/3 cup buttermilk
2 tablespoons minced fresh flat-leaf parsley
2 tablespoons minced fresh chives
1 scallion (white and green parts), thinly sliced
1 teaspoon white wine vinegar
Freshly ground black pepper
Smash the garlic clove, sprinkle with the salt, and, with the side of a large knife, mash and smear the mixture to a coarse paste. Scrape the paste into a small bowl, add the remaining ingredients, and whisk well to make a creamy dressing. Use immediately or refrigerate in a tightly sealed container for up to 3 days.
Yield: about 1 1/3 cups
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