This potato salad uses spring onions two ways — pickled, and in a vibrant pesto. Any small potato (red new potatoes, or fingerlings) would work here, but if you ask Alexandre Dumas, author of The Count of Monte Cristo and an avid fan of the potato, "the best, without question, are the purplish ones, known in Paris by the name vitelottes." While you might not notice the taste difference he did, it's easy to see why he liked them: They transform the humble potato into the visual showstopper he hoped they would become.
In a small bowl, combine the vinegar, salt, sugar, and water, stirring until the sugar has dissolved. Thinly slice the whites of the spring onions, and submerge in the vinegar mixture. Set aside.
Place the potatoes in a large pot and cover with water until just submerged. Bring to a boil and cook until tender when pierced with a fork, about 10 minutes. Drain the potatoes and set aside.
Coarsely chop the onion greens and place in the bowl of a food processor, along with the pine nuts, Parmesan, and garlic. Pulse until a thick paste forms. While the food processor is running, add the olive oil in a slow stream until the pesto is smooth.
Place the potatoes in a large bowl and fold in the pesto until the potatoes are coated. Drain the onion whites and add to the bowl, along with the parsley, lemon juice, and nutmeg. Toss until combined. Season with salt and pepper, to taste.