Place the potatoes in a pot of cold water. Bring to a boil and cook until tender to a fork. These took different times as the purple potatoes were much smaller. In general, the potatoes were fork-tender within 15-25 minutes. Drain the potatoes in a colander. As they cool, make the vinagrette.
Put the lemon juice, dijon mustard, olive oil, salt and pepper in a little jar that has a lid. I used about 1 teaspoon of salt and 1/2 teaspoon of freshly ground black pepper. Put the lid on the jar and shake until well combined.
Chop or slice the potatoes into your desired shape. I cut them into quarters. In a large bowl, place the potatoes, scapes, onion, cilantro and lemon zest. Pour the vinaigrette over the top and watch as the warm potatoes suck up the tangy dressing. Toss everything very well and leave to marinate at room temperature until the rest of your meal is ready to be served. Taste just before serving and re-season, as needed, with salt and pepper.