Bring a pot of water (enough to cover the potatoes when you add them) to a boil on the stovetop. Add potatoes, bring the water back to a boil, then reduce heat and simmer until potatoes are tender (10 to 15 minutes).

While potatoes are cooking, whip up your vinaigrette. In a small bowl or jar with a tightly fitting lid, combine lemon juice, mustard, sugar, and salt.

Add olive oil and whisk or shake vigorously until well combined. Taste and add more sugar and/or salt if needed.

When potatoes are ready, drain and let them cool for just a couple of minutes. (While this salad is best served warm, you don't want to burn yourself.)

In a large serving bowl, toss the warm potatoes with the vinaigrette, celery, green onions, mint, and capers.